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Document And Entity Information - shares	3 Months Ended
Document And Entity Information - shares	Mar. 31, 2019	Apr. 30, 2019
Document And Entity Information [Abstract]
Entity Registrant Name	Kaleido BioSciences, Inc.
Trading Symbol	KLDO
Entity Central Index Key	0001751299
Current Fiscal Year End Date	--12-31
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		29,807,703
Emerging Growth Company	true
Ex Transition Period	false
Entity Small Business	false

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 121,340	$ 76,086
Prepaid expenses and other current assets	2,643	152
Due from related party	0	5
Total current assets	123,983	76,243
Property and equipment, net	5,103	4,693
Restricted cash	2,182	2,180
Deferred issuance costs	0	2,209
Total assets	131,268	85,325
Current liabilities:
Accounts payable	3,575	3,442
Accrued expenses and other current liabilities	5,274	7,859
Total current liabilities	8,849	11,301
Long term debt, net of unamortized debt discount	14,844	14,831
Restricted shares repurchase liability	394	720
Other liabilities	45	278
Warrant liability	0	1,213
Total liabilities	24,132	28,343
Redeemable convertible preferred stock, $0.001 par value, no shares and 37,171,832 shares authorized; no shares and 37,034,802 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively; liquidation preference of $153,541 at December 31, 2018	0	153,226
Commitments and contingencies (Note 8)
Shareholders’ equity:
Preferred shares, $0.001 par value, 10,000,000 and no shares authorized; no shares issued or outstanding at March 31, 2019 and December 31, 2018, respectively	0	0
Common shares, $0.001 par value, 150,000,000 and 66,000,000 shares authorized; 29,759,249 and 6,115,535 shares issued; 29,579,468 and 5,786,911 shares outstanding at March 31, 2019 and December 31, 2018, respectively	30	6
Additional paid-in capital	233,553	9,978
Accumulated deficit	(126,447)	(106,228)
Total shareholders' equity (deficit)	107,136	(96,244)
Total liabilities, redeemable convertible preferred stock and shareholders’ (deficit) equity	$ 131,268	$ 85,325

Condensed Consolidated Balance Sheets (Unaudited) - Parenthetical - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock par value (in dollars per share)	$ 0.001	$ 0.001
Redeemable convertible preferred stock shares authorized (in shares)	37,171,832	37,171,832
Redeemable convertible preferred stock shares issued (in shares)	0	37,034,802
Redeemable convertible preferred stock shares outstanding (in shares)	0	37,034,802
Redeemable convertible preferred stock liquidation preference		$ 153,541
Preferred Stock
Preferred shares par value (in dollars per share)	$ 0.001	$ 0.001
Preferred shares authorized (in shares)	10,000,000	0
Preferred shares issued (in shares)	0	0
Preferred shares outstanding (in shares)	0	0
Common Stock
Common shares par value (in dollars per share)	$ 0.001	$ 0.001
Common shares authorized (in shares)	150,000,000	66,000,000
Common shares issued (in shares)	29,759,249	6,115,535
Common shares outstanding (in shares)	29,579,468	5,786,911

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 15,182	$ 7,546
General and administrative	5,433	2,670
Total operating expenses	20,615	10,216
Loss from operations	(20,615)	(10,216)
Other (expense) income:
Interest income	407	70
Interest expense	(258)	(249)
Change in fair value of warrant liability	252	(350)
Other expense	(5)	(15)
Total other expense, net	396	(544)
Net loss	$ (20,219)	$ (10,760)
Net loss per share attributable to common shareholders—basic and diluted (in dollars per share)	$ (1.56)	$ (2.27)
Weighted-average common shares outstanding used in net loss per share attributable to common shareholders—basic and diluted (in shares)	12,963,994	4,743,824

Condensed Consolidated Statements of Stockholders’ (Deficit) Equity (Unaudited) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		26,927,398
Beginning balance at Dec. 31, 2017		$ 52,494
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series C convertible preferred stock (net of issuance costs of $168) (in shares)		7,116,414
Issuance of Series C convertible preferred stock (net of issuance costs of $168)		$ 70,925
Ending balance (in shares) at Mar. 31, 2018		34,043,812
Ending balance at Mar. 31, 2018		$ 123,419
Beginning balance (in shares) at Dec. 31, 2017			4,711,963
Beginning balance at Dec. 31, 2017	$ (43,679)		$ 5	$ 800	$ (44,484)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering	0
Exercise of stock options (in shares)			9,944
Exercise of stock options	2			2
Stock-based compensation	234			234
Vesting of restricted shares (in shares)			72,656
Vesting of restricted shares	158			158
Net loss	(10,760)				(10,760)
Beginning balance (in shares) at Mar. 31, 2018			4,794,563
Beginning balance at Mar. 31, 2018	$ (54,045)		$ 5	1,194	(55,244)
Beginning balance (in shares) at Dec. 31, 2018	37,034,802	37,034,802
Beginning balance at Dec. 31, 2018	$ 153,226	$ 153,226
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of redeemable convertible preferred stock into common stock (in shares)		(37,034,802)
Conversion of redeemable convertible preferred stock into common stock		$ (153,226)
Ending balance (in shares) at Mar. 31, 2019	0	0
Ending balance at Mar. 31, 2019	$ 0	$ 0
Beginning balance (in shares) at Dec. 31, 2018			5,786,911
Beginning balance at Dec. 31, 2018	(96,244)		$ 6	9,978	(106,228)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of redeemable convertible preferred stock into common stock (in shares)			18,517,386
Conversion of redeemable convertible preferred stock into common stock	153,226		$ 19	153,207
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering	871			871
Issuance of common stock, net of issuance costs of $8,411 (in shares)			5,000,000
Issuance of common stock, net of issuance costs of $8,411	$ 66,589		$ 5	66,584
Exercise of common stock warrant (in shares)	51,015		51,015
Exercise of common stock warrant	$ 0
Exercise of stock options (in shares)	75,313		75,313
Exercise of stock options	$ 60			60
Stock-based compensation	2,528			2,528
Vesting of restricted shares (in shares)			148,843
Vesting of restricted shares	325			325
Net loss	(20,219)				(20,219)
Beginning balance (in shares) at Mar. 31, 2019			29,579,468
Beginning balance at Mar. 31, 2019	$ 107,136		$ 30	$ 233,553	$ (126,447)

Condensed Consolidated Statements of Stockholders' (Deficit) Equity (Unaudited) - Parenthetical - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Stockholders' Equity [Abstract]
Common stock, issuance costs	$ 8,411
Issuance costs of Series C convertible preferred stock		$ 168

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Operating activities:
Net loss	$ (20,219)	$ (10,760)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	293	156	$ 156
Equity-based compensation	2,528	234
Non-cash interest expense	12	21
Change in fair value of warrant liability	(252)	350
Changes in:
Prepaid expenses and other assets	(2,486)	(249)
Accounts payable	(129)	777
Accrued expense and other liabilities	(2,543)	13
Net cash used in operating activities	(22,796)	(9,458)
Investing activities:
Purchase of property and equipment	(1,126)	(278)
Net cash and restricted cash used in investing activities	(1,126)	(278)
Financing activities:
Proceeds from preferred stock financing, net of issuance costs	0	70,925
Proceeds from exercise of stock options	60	2
Payments related to capital lease	(25)	(26)
Issuance of common stock, net of issuance costs	69,443	0
Settlement of derivative liability	(300)	0
Net cash provided by financing activities	69,178	70,901
Net increase in cash, cash equivalents, and restricted cash	45,256	61,165
Cash, cash equivalents, and restricted cash, beginning of period	78,266	28,677	28,677
Cash, cash equivalents, and restricted cash, end of period	123,522	89,842	$ 78,266
Supplemental cash flow information
Interest paid	245	228
Supplemental disclosure of non-cash investing and financing activities
Vesting of restricted stock	325	158
Reclassification of warrants to additional paid-in capital	871	0
Conversion of preferred stock to common stock upon closing of the initial public offering	153,226	0
Initial public offering costs incurred but unpaid at period end	1,039	0
Purchase of property and equipment in accounts payable and accrued expenses	$ 181	$ 367

Nature of the Business and Basis of Presentation	3 Months Ended
Nature of the Business and Basis of Presentation	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Kaleido Biosciences, Inc. and its wholly owned subsidiaries, Cadena Bio, Inc. and Kaleido Biosciences Securities Corporation (collectively referred to as the “Company”) is a clinical-stage healthcare company that was incorporated in Delaware on January 27, 2015 and has a principal place of business in Lexington, Massachusetts. The Company was formed to use its differentiated, chemistry-driven approach to leverage the potential of the microbiome organ to treat disease and improve human health. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, successful development of technology, obtaining additional funding, protection of proprietary technology, compliance with government regulations, risks of failure of preclinical studies (including ex-vivo assays), non-IND human clinical studies and clinical trials, the need to obtain marketing approval for its drug candidates and its consumer products, fluctuations in operating results, economic pressure impacting therapeutic pricing, dependence on key personnel, risks associated with changes in technologies, development by competitors of technological innovations and the ability to supply sufficient amounts of Microbiome Metabolic Therapies ("MMTs") at an acceptable quality level. On March 4, 2019, the Company completed its initial public offering (the "IPO"), pursuant to which it issued and sold 5,000,000 shares of common stock. The aggregate net proceeds received by the Company from the IPO were $69,750, after deducting underwriting discounts and commissions, but before deducting offering costs payable by the Company, which are estimated to be $3,161, as of March 31, 2019. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 18,517,386 shares of common stock (see Note 9). The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has financed its operations through private placements of its equity securities, borrowings of long-term debt and most recently, through a public offering. As of March 31, 2019, the Company had an accumulated deficit of $126,447. The Company expects to continue to generate operating losses in the foreseeable future. As of the issuance date of the consolidated financial statements, the Company expects that its cash and cash equivalents, including net proceeds it received from the completion of the IPO, will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance date of the interim consolidated financial statements.

Summary of Significant Accounting Policies	3 Months Ended
Summary of Significant Accounting Policies	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Unaudited interim financial information The consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP") have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Prospectus that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-229204), which was filed with the SEC pursuant to Rule 424(b)(4) on March 1, 2019 (the “Prospectus”). All intercompany transactions and balances of the subsidiary have been eliminated in consolidation. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair representation of the results for the reported interim periods. Use of Estimates The preparation of the consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting period. Actual results could differ from those estimates. Summary of Significant Accounting Policies The Company’s significant accounting policies are described in Note 2, “Summary of Significant Accounting Policies,” in the Prospectus. There have been no material changes to the significant accounting policies during the period ended March 31, 2019. Accounting Pronouncements Issued and Not Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (“ASU 2016-02”), which applies to all leases and will require lessees to record most leases on the balance sheet but recognize expense in a manner similar to the current standard. The Company will use a modified retrospective approach of adoption for leases that exist or are entered into after the beginning of the earliest comparative period in the financial statements. The Company is currently evaluating whether to early-adopt ASU 2016-02 and ASU No. 2018-11 and evaluating the impact that the adoption of ASU 2016-02 and ASU No. 2016-02 will have on its consolidated financial statements. The Company expects to recognize a significant lease obligation and right to use asset upon adoption.

Fair Value Measurements

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

The following tables set forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis:



	Fair Value Measurements as of March 31, 2019
	Level 1		Level 2	Level 3	Total
Assets:
Money market funds included within cash and cash equivalents	$	116,548	—	—	$	116,548
Total	$	116,548	—	—	116,548



	Fair Value Measurements as of December 31, 2018
	Level 1		Level 2	Level 3	Total
Assets:
Money market funds included within cash and cash equivalents	$	74,145	—	—	$	74,145
Total	$	74,145	—	—	$	74,145

Liabilities:
Warrant liability	$	—	—	1,213	$	1,213
Derivative liability	—		—	210	$	210
	$	—	—	1,423	$	1,423

The fair value of money market funds was measured by the Company based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy.

The preferred stock warrant liability consisted of the fair value of warrants to purchase Series A and Series B convertible preferred stock (see Note 7) and was based on significant inputs not observable in the market, which represent a Level 3 measurement within the fair value hierarchy. The Company’s valuation of the preferred stock warrants utilized the Black-Scholes option-pricing model, which incorporates assumptions and estimates to value the preferred stock warrants. The Company assesses these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Changes in the fair value of the preferred stock warrants are recognized as other income (expense) in the consolidated statements of operations.

The quantitative elements associated with the Company’s Level 3 inputs that impact the fair value measurement of the preferred stock warrant liability include the fair value per share of the underlying Series A and Series B convertible preferred stock, the remaining contractual term of the warrants, risk-free interest rate, expected dividend yield and expected volatility of the price of the underlying preferred stock. The most significant assumption in the Black-Scholes option-pricing model that impacts the fair value of the preferred stock warrants is the fair value of the Company’s convertible preferred stock as of each re-measurement date. The Company determines the fair value per share of the underlying preferred stock by taking into consideration its most recent sales of its convertible preferred stock as well as additional factors that the Company deems relevant. The Company historically has been a private company and lacks company-specific historical and implied volatility information of its stock. Therefore, it estimates its expected stock volatility based on the historical volatility of publicly-traded peer companies for a term equal to the remaining contractual term of the warrants. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrants. The Company has estimated a 0% dividend yield based on the expected dividend yield and the fact that the Company has never paid or declared dividends.

Upon the closing of the IPO, the warrants for the purchase of preferred stock automatically became warrants for the purchase of common stock and the Company reclassified the carrying value of the warrants from a non-current liability to additional paid-in capital in its consolidated balance sheet.

The fair value of the derivative liability recognized in connection with the contingent success fee associated with the amended term loan agreement (see Note 6) was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the derivative liability was determined using the probability-weighted expected return method (“PWERM”), which considered as inputs the probability of occurrence of an event (as defined), the expected timing of a liquidity event, the amount of the success fee and a risk-adjusted discount rate. As of December 31, 2018, the assumed probability of occurrence of the event that was most probable of triggering the payment was 70%, the expected timing of such an event was estimated to be less than one year, the amount of the success fee was $300 and the discount rate was assessed to be 0%. As of March 4, 2019, the closing date of the IPO, the assumed probability of occurrence of the event that was most probable of triggering the payment increased to 100% and the discount rate was assessed to be 0%. Based on these inputs, the Company determined that the fair value of the derivative liability was $210 as of December 31, 2018 and $300 as of March 4, 2019. Upon completion of the IPO, the success fee of $300 was paid in March 2019.

The following table provides a roll-forward of the aggregate fair values of the Company’s preferred stock warrants for which fair value was determined by Level 3 inputs (in thousands):



	Warrant Liability		Derivative Liability
Fair value at January 1, 2019	1,213		210
Change in fair value through the exercise date	(342	)	90
Reclassification to additional paid-in-capital in connection with IPO	(871	)	—
Settlement of derivative instrument	—		(300	)
Fair value at March 31, 2019	—		—

There were no transfers among the Level 1, Level 2, or Level 3 categories in the periods presented.

Financial Instruments Not Recorded at Fair Value – The carrying value of cash, cash equivalents, restricted cash, accounts payable and accrued expenses that are reported on the consolidated balance sheets approximate their fair value due to the short-term nature of these assets and liabilities. The carrying value of the long-term debt approximates fair value as evidenced by the recent amendment to the term loan agreement.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment, net

3 Months Ended

Mar. 31, 2019

Property, Plant and Equipment [Abstract]

Property and Equipment, net

Property and equipment consist of the following:



	As of
	March 31, 2019			December 31, 2018
Laboratory equipment	$	3,503		$	3,226
Office and computer equipment	1,376			1,337
Leasehold improvements	653			653
Construction in process	1,085			698
Property and equipment – at cost	6,617			5,914
Less accumulated depreciation and amortization	(1,514		)	(1,221		)
Property and equipment – net	$	5,103		$	4,693

Depreciation and amortization expense for the three months ended March 31, 2019 and 2018 was $293 and $156, respectively.

Accrued Expenses and Other Current Liabilities

3 Months Ended

Mar. 31, 2019

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:



	As of
	March 31, 2019		December 31, 2018
Payroll and benefits	$	1,213	$	3,297
Consulting service	467		243
Legal service	180		90
Research and development	2,389		3,718
Capital lease payable – short term	88		91
Other	937		420
	$	5,274	$	7,859

Debt Financing	3 Months Ended
Debt Financing	Mar. 31, 2019
Debt Disclosure [Abstract]
Debt Financing	Debt Financing The Company is party to a loan and security agreement, as amended (the “2015 Credit Facility”), under which the Company has borrowed an aggregate of $15,000. Borrowings under the 2015 Credit Facility bear interest at an annual rate equal to the lender’s prime rate plus 1.00%, subject to a floor of 5.75%, and are repayable in monthly interest-only payments through April 2020 and in equal monthly payments of principal plus accrued interest from May 2020 until the maturity date in April 2022. As of March 31, 2019, the interest rate applicable to borrowings under the 2015 Credit Facility was 6.50%. Borrowings under the 2015 Credit Facility are collateralized by substantially all of the Company’s personal property, other than its intellectual property. There are no financial covenants associated with the 2015 Credit Facility; however, the Company is subject to certain affirmative and negative covenants restricting the Company’s activities, including limitations on dispositions, mergers or acquisitions; encumbering its intellectual property; incurring indebtedness or liens; paying dividends; making certain investments; and engaging in certain other business transactions. The obligations under the 2015 Credit Facility are subject to acceleration upon the occurrence of specified events of default, including a material adverse change in the Company’s business, operations or financial or other condition. The scheduled principal maturity of the long-term debt is $5,000 in 2020, $7,500 in 2021 and $2,500 in 2022. As part of an amendment to the 2015 Credit Facility in June 2018, a success fee of $300 was required in the event of a liquidation event, including an IPO. The success fee represented an embedded derivative which the Company bifurcated from the debt arrangement and carried at fair value. In March 2019, the Company completed its IPO and paid the success fee of $300.

Warrant Liability	3 Months Ended
Warrant Liability	Mar. 31, 2019
Warrants and Rights Note Disclosure [Abstract]
Warrant Liability	Warrant Liability In 2015, the Company issued warrants to purchase up to 85,617 shares of Series A Preferred Stock in connection with the 2015 Credit Facility (see Note 6). The warrants were exercisable at a price of $0.73 per share and had a contractual term of ten years from issuance. In October 2017, the Company issued warrants to purchase up to 51,413 shares of Series B Preferred Stock in connection with an amendment to the 2015 Credit Facility (see Note 6). The warrants were exercisable at a price of $3.89 per share and had a contractual term of ten years from issuance. The Company remeasured the fair value of the liability for these preferred stock warrants at each reporting date and recorded any adjustments as other income (expense). The warrants outstanding at each reporting date were remeasured using the Black-Scholes option-pricing model (see Note 3), and the resulting change in fair value was recorded in other income (expense) in the Company’s consolidated statements of operations. Upon the closing of the IPO in March 2019, the Company’s outstanding warrants to purchase Series A Preferred Stock and Series B Preferred Stock automatically became warrants to purchase an aggregate of 68,514 shares of common stock. In March 2019, the holders of such warrants completed a cashless exercise of the warrants, resulting in the Company’s issuance of 51,015 shares of common stock.

Commitments and contingencies

3 Months Ended

Mar. 31, 2019

Commitments and Contingencies Disclosure [Abstract]

Commitments and contingencies

Facilities Leases

Lexington, MA Lease

In March 2018, the Company entered into a non-cancelable ten-year lease agreement for laboratory and office space in Lexington, Massachusetts. In March 2019, the Company exercised its option to lease additional premises (the “Expansion Premises”) comprised of 54,468 square feet of space (the entire second floor of the building). Under the terms of the option, the Company will not gain access to the additional space until December 1, 2019 at the earliest.

Rent expense for the three months ended March 31, 2019 and 2018 totaled $1,024 and $245, respectively. Future minimum lease payments under the non-cancelable operating leases consisted of the following as of March 31, 2019:



Year Ending December 31,
2019	$	2,794
2020	5,757
2021	5,922
2022	6,100
2023	6,283
Thereafter	37,155
	$	64,011

Common stock	3 Months Ended
Common stock	Mar. 31, 2019
Equity [Abstract]
Common stock	Common stock Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors. Upon the closing of the IPO in March 2019, all of the shares of the Company’s outstanding convertible preferred stock automatically converted into 18,517,386 shares of common stock. In March 2019, the Company filed a restated certificate of incorporation in the State of Delaware, which, among other things, restated the number of shares of all classes of stock that the Company has authority to issue to 160,000,000 shares, consisting of (i) 150,000,000 shares of common stock, $0.001 par value per share, and (ii) 10,000,000 shares of preferred stock, $0.001 par value per share. The shares of preferred stock are currently undesignated.

Stock-based compensation

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-based compensation

2015 Stock Incentive Plan

The Company’s 2015 Stock Incentive Plan (the “2015 Plan”) provided for the Company to sell or issue incentive stock options or nonqualified stock options, restricted stock, and other equity awards to employees, directors and consultants of the Company. The 2015 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

The total number of shares of common stock that could have been issued under the 2015 Plan was 8,395,974 shares, of which 10,558 shares remained available for future issuance prior to the effectiveness of the Company’s 2019 Stock Option and Incentive Plan (the “2019 Plan”). Upon effectiveness of the 2019 Plan, the remaining shares available under the 2015 Plan ceased to be available for issuance and no future issuances will be made under the 2015 Plan. The shares of common stock underlying outstanding awards under the 2015 Plan that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) will be added to the shares of common stock available for issuance under the 2019 Plan.

2019 Stock Option and Incentive Plan

The 2019 Plan was adopted by the Company’s board of directors on January 23, 2019, and approved by stockholders on February 19, 2019, and became effective on February 27, 2019. The 2019 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company’s officers, employees, directors and consultants. The number of shares initially reserved for issuance under the 2019 Plan is 2,168,976, which shall be cumulatively increased on January 1, 2020 and each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors.

2019 Employee Stock Purchase Plan

The 2019 Employee Stock Purchase Plan (the “2019 ESPP”) was adopted by the Company’s board of directors on January 23, 2019, and adopted by stockholders on February 19, 2019, and became effective on February 27, 2019. The 2019 ESPP has not been implemented as of March 31, 2019. A total of 180,748 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on January 1, 2020, and each January 1 thereafter, by the lesser of (i) 542,244 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the administrator of the 2019 ESPP.

Stock Option Valuation

The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The assumptions that the Company used to determine the grant-date fair value of options granted to employees, non-employees, and directors were as follows:



	Three Months Ended March 31,
	2019
Expected volatility	66.1%-71.1%
Risk-free interest rate	2.27%-2.54%
Expected term (in years)	6.00-6.25
Expected dividend yield	0%

The Company did not grant options during the three months ended March 31, 2018.

Stock Options Activity

A summary of the Company’s stock option activity and related information is as follows:



	Options		Weighted Average Exercise Price		Weighted Average Remaining Life (in Years)	Aggregate Intrinsic Value
Outstanding as of January 1, 2019	6,686,267		$	7.50	9.2	68,167
Granted	351,500		14.90
Exercised	(75,313	)	0.75
Canceled	(24,748	)	7.13
Outstanding as of March 31, 2019	6,937,706		$	7.95	9.0	47,698
Options exercisable as of March 31, 2019	677,605		$	2.05	7.9	8,433
Options vested or expected to vest- March 31, 2019	6,937,706		$	7.95	9.0	47,698

The weighted-average grant date fair value of the options granted during the three months ended March 31, 2019 was $9.35 per share. As of March 31, 2019 there was $31,575 of unrecognized compensation expense, which the Company expects to recognize over the weighted-average remaining term of 3.16 years.

Restricted Common Stock

During the year ended December 31, 2017, the Company signed agreements with seven employees to early exercise stock options covering 1,295,699 shares to convert such options to restricted common stock prior to the vesting of the underlying shares of common stock. The vesting conditions did not change. The consideration received due to the early exercises from the seven employees was recorded as a restricted share repurchase liability. As of March 31, 2019 and December 31, 2018, the outstanding balance of the restricted share repurchase liability was $394 and $720, respectively.

The following table summarizes the Company’s restricted common stock activity for the period ended March 31, 2019:



	Number of Restricted Shares		Weighted-Average Grant Date Fair Value
Issued and unvested as of January 1, 2019	328,624		$	2.19
Vested	(148,843	)
Issued and unvested as of March 31, 2019	179,781		$	2.19

Stock- Based Compensation Expense

The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations:



	Three Months Ended March 31,
	2019		2018
Research and development	$	873	$	86
General and administrative	1,655		148
	$	2,528	$	234

Net Loss per Share

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Net Loss per Share

Basic and diluted net loss per common share is determined by dividing net loss by the weighted-average common shares outstanding during the period. The Company has computed diluted net loss per common share after giving consideration to all potentially dilutive common shares, including options to purchase common stock, restricted common stock, convertible preferred stock and warrants to purchase convertible preferred stock, outstanding during the period determined using the if-converted and treasury stock methods, except where the effect of including such securities would be anti-dilutive. Because the Company has reported net losses since inception, these potential common shares have been anti-dilutive and therefore basic and diluted net loss per share have been equivalent.

The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive:



	As of March 31,
	2019	2018
Options to purchase common stock	6,937,706	2,765,623
Unvested restricted common stock	179,781	1,214,762
Convertible redeemable preferred stock (as converted to common stock)	—	17,021,894
Warrant to purchase redeemable convertible preferred stock (as converted to common stock)	—	68,514
	7,117,487	21,070,793

Related Party Transactions	3 Months Ended
Related Party Transactions	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions The Company receives professional services from its principal investor, Flagship Pioneering, from time to time as needed. The Company reported general and administrative expense totaling $8 and $62 related to these services for the three months ended March 31, 2019 and 2018, respectively.

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events As of May 2, 2019, the Company is not aware of any events that have occurred that have a material effect on the financial statements.

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2019
Accounting Policies [Abstract]
Consolidation	All intercompany transactions and balances of the subsidiary have been eliminated in consolidation.
Use of Estimates	The preparation of the consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting period. Actual results could differ from those estimates.
Accounting Pronouncements Issued and Not Adopted	In February 2016, the FASB issued ASU No. 2016-02, Leases (“ASU 2016-02”), which applies to all leases and will require lessees to record most leases on the balance sheet but recognize expense in a manner similar to the current standard. The Company will use a modified retrospective approach of adoption for leases that exist or are entered into after the beginning of the earliest comparative period in the financial statements. The Company is currently evaluating whether to early-adopt ASU 2016-02 and ASU No. 2018-11 and evaluating the impact that the adoption of ASU 2016-02 and ASU No. 2016-02 will have on its consolidated financial statements. The Company expects to recognize a significant lease obligation and right to use asset upon adoption.
Net Loss per Share	Basic and diluted net loss per common share is determined by dividing net loss by the weighted-average common shares outstanding during the period. The Company has computed diluted net loss per common share after giving consideration to all potentially dilutive common shares, including options to purchase common stock, restricted common stock, convertible preferred stock and warrants to purchase convertible preferred stock, outstanding during the period determined using the if-converted and treasury stock methods, except where the effect of including such securities would be anti-dilutive. Because the Company has reported net losses since inception, these potential common shares have been anti-dilutive and therefore basic and diluted net loss per share have been equivalent.

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Carried at Fair Value on a Recurring Basis

The following tables set forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis:



	Fair Value Measurements as of March 31, 2019
	Level 1		Level 2	Level 3	Total
Assets:
Money market funds included within cash and cash equivalents	$	116,548	—	—	$	116,548
Total	$	116,548	—	—	116,548



	Fair Value Measurements as of December 31, 2018
	Level 1		Level 2	Level 3	Total
Assets:
Money market funds included within cash and cash equivalents	$	74,145	—	—	$	74,145
Total	$	74,145	—	—	$	74,145

Liabilities:
Warrant liability	$	—	—	1,213	$	1,213
Derivative liability	—		—	210	$	210
	$	—	—	1,423	$	1,423

Roll-Forward of Aggregate Fair Values of Liabilities Using Level 3 Inputs

The following table provides a roll-forward of the aggregate fair values of the Company’s preferred stock warrants for which fair value was determined by Level 3 inputs (in thousands):



	Warrant Liability		Derivative Liability
Fair value at January 1, 2019	1,213		210
Change in fair value through the exercise date	(342	)	90
Reclassification to additional paid-in-capital in connection with IPO	(871	)	—
Settlement of derivative instrument	—		(300	)
Fair value at March 31, 2019	—		—

Property and Equipment, net (Tables)

3 Months Ended

Mar. 31, 2019

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consist of the following:



	As of
	March 31, 2019			December 31, 2018
Laboratory equipment	$	3,503		$	3,226
Office and computer equipment	1,376			1,337
Leasehold improvements	653			653
Construction in process	1,085			698
Property and equipment – at cost	6,617			5,914
Less accumulated depreciation and amortization	(1,514		)	(1,221		)
Property and equipment – net	$	5,103		$	4,693

Accrued Expenses and Other Current Liabilities (Tables)

3 Months Ended

Mar. 31, 2019

Payables and Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:



	As of
	March 31, 2019		December 31, 2018
Payroll and benefits	$	1,213	$	3,297
Consulting service	467		243
Legal service	180		90
Research and development	2,389		3,718
Capital lease payable – short term	88		91
Other	937		420
	$	5,274	$	7,859

Commitments and contingencies (Tables)

3 Months Ended

Mar. 31, 2019

Commitments and Contingencies Disclosure [Abstract]

Future Minimum Lease Payments

Future minimum lease payments under the non-cancelable operating leases consisted of the following as of March 31, 2019:



Year Ending December 31,
2019	$	2,794
2020	5,757
2021	5,922
2022	6,100
2023	6,283
Thereafter	37,155
	$	64,011

Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Assumptions to Determine Grant-Date Fair Value of Options

The assumptions that the Company used to determine the grant-date fair value of options granted to employees, non-employees, and directors were as follows:



	Three Months Ended March 31,
	2019
Expected volatility	66.1%-71.1%
Risk-free interest rate	2.27%-2.54%
Expected term (in years)	6.00-6.25
Expected dividend yield	0%

Stock Option Activity

A summary of the Company’s stock option activity and related information is as follows:



	Options		Weighted Average Exercise Price		Weighted Average Remaining Life (in Years)	Aggregate Intrinsic Value
Outstanding as of January 1, 2019	6,686,267		$	7.50	9.2	68,167
Granted	351,500		14.90
Exercised	(75,313	)	0.75
Canceled	(24,748	)	7.13
Outstanding as of March 31, 2019	6,937,706		$	7.95	9.0	47,698
Options exercisable as of March 31, 2019	677,605		$	2.05	7.9	8,433
Options vested or expected to vest- March 31, 2019	6,937,706		$	7.95	9.0	47,698

Restricted Common Stock Activity

The following table summarizes the Company’s restricted common stock activity for the period ended March 31, 2019:



	Number of Restricted Shares		Weighted-Average Grant Date Fair Value
Issued and unvested as of January 1, 2019	328,624		$	2.19
Vested	(148,843	)
Issued and unvested as of March 31, 2019	179,781		$	2.19

Allocated Stock-based Compensation Expense

The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations:



	Three Months Ended March 31,
	2019		2018
Research and development	$	873	$	86
General and administrative	1,655		148
	$	2,528	$	234

Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Schedule of Securities Excluded from Computation of Diluted Weighted-Average Shares Outstanding

The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive:



	As of March 31,
	2019	2018
Options to purchase common stock	6,937,706	2,765,623
Unvested restricted common stock	179,781	1,214,762
Convertible redeemable preferred stock (as converted to common stock)	—	17,021,894
Warrant to purchase redeemable convertible preferred stock (as converted to common stock)	—	68,514
	7,117,487	21,070,793

Nature of the Business and Basis of Presentation (Details) - USD ($) $ in Thousands		3 Months Ended
	Mar. 04, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Conversion of Stock [Line Items]
Proceeds from initial public offering		$ 69,443	$ 0
Offering costs payable		1,039	$ 0
Accumulated deficit		126,447		$ 106,228
IPO
Conversion of Stock [Line Items]
Issuance of common stock (in shares)	5,000,000
Proceeds from initial public offering	$ 69,750
Offering costs payable		$ 3,161
Common Stock
Conversion of Stock [Line Items]
Conversion of stock, shares converted (in shares)	18,517,386	18,517,386

Fair Value Measurements - Schedule of Assets and Liabilities Carried at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability	$ 0	$ 1,213
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	116,548	74,145
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability		1,213
Derivative liability		210
Liabilities, fair value disclosure		1,423
Fair Value, Measurements, Recurring \| Level 1
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	116,548	74,145
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability		0
Derivative liability		0
Liabilities, fair value disclosure		0
Fair Value, Measurements, Recurring \| Level 2
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability		0
Derivative liability		0
Liabilities, fair value disclosure		0
Fair Value, Measurements, Recurring \| Level 3
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Warrant liability		1,213
Derivative liability		210
Liabilities, fair value disclosure		1,423
Money market funds included within cash and cash equivalents \| Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	116,548	74,145
Money market funds included within cash and cash equivalents \| Fair Value, Measurements, Recurring \| Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	116,548	74,145
Money market funds included within cash and cash equivalents \| Fair Value, Measurements, Recurring \| Level 2
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	0	0
Money market funds included within cash and cash equivalents \| Fair Value, Measurements, Recurring \| Level 3
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	$ 0	$ 0

Fair Value Measurements - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended
	Mar. 04, 2019 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Measurements, Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability				$ 210
Fair Value, Measurements, Recurring \| Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability				210
Valuation Technique, Option Pricing Model \| Fair Value, Measurements, Recurring \| Level 3 \| Measurement Input, Dividend Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant liability, measurement input		0.00	0.00
Valuation Technique, Probability-Weighted Expected Return Method \| Fair Value, Measurements, Recurring \| Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, term	1 year
Derivative liability	$ 300			$ 210
Valuation Technique, Probability-Weighted Expected Return Method \| Fair Value, Measurements, Recurring \| Level 3 \| Measurement Input, Probability Of Liquidity Event
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	1.00			0.70
Valuation Technique, Probability-Weighted Expected Return Method \| Fair Value, Measurements, Recurring \| Level 3 \| Measurement Input, Success Fee
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability				$ 300
Valuation Technique, Probability-Weighted Expected Return Method \| Fair Value, Measurements, Recurring \| Level 3 \| Measurement Input, Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.00			0.00
Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Success fee		$ 300	$ 300

Fair Value Measurements - Roll-Forward of Aggregate Fair Values of Liabilities Using Level 3 Inputs (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2019
Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at January 1, 2019		$ 1,213
Change in fair value through the exercise date		(342)
Reclassification to additional paid-in-capital in connection with IPO		(871)
Settlement of derivative instrument		0
Fair value at March 31, 2019	$ 0	0
Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at January 1, 2019		210
Change in fair value through the exercise date		90
Reclassification to additional paid-in-capital in connection with IPO		0
Settlement of derivative instrument	(300)	(300)
Fair value at March 31, 2019	$ 0	$ 0

Property and Equipment, net (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment – at cost	$ 6,617		$ 5,914
Less accumulated depreciation and amortization	(1,514)		(1,221)
Property and equipment – net	5,103		4,693
Depreciation and amortization	293	$ 156	156
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment – at cost	3,503		3,226
Office and computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment – at cost	1,376		1,337
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment – at cost	653		653
Construction in process
Property, Plant and Equipment [Line Items]
Property and equipment – at cost	$ 1,085		$ 698

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Payroll and benefits	$ 1,213	$ 3,297
Consulting service	467	243
Legal service	180	90
Research and development	2,389	3,718
Capital lease payable – short term	88	91
Other	937	420
Total accrued expenses and other current liabilities	$ 5,274	$ 7,859

Debt Financing (Details)	3 Months Ended
Debt Financing (Details)	Mar. 31, 2019 USD ($)
Line of Credit Facility [Line Items]
Maturity of long term debt due 2020	$ 5,000,000
Maturity of long term debt due 2021	7,500,000
Maturity of long term debt due 2022	2,500,000
2015 Credit Facility \| Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 15,000
Debt instrument, variable rate, floor	5.75%
Interest rate at end of period	6.50%
Success fee	$ 300,000
2015 Credit Facility \| Line of Credit \| Prime Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	1.00%

Warrant Liability (Details) - $ / shares	3 Months Ended
Warrant Liability (Details) - $ / shares	Mar. 31, 2019	Oct. 31, 2017	Dec. 31, 2015
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)	68,514
Number of shares issued in exercised of warrants (in shares)	51,015
Series A Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)			85,617
Warrants, exercise price per share (in dollars per share)			$ 0.73
Warrants contractual term			10 years
Series B Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)		51,413
Warrants, exercise price per share (in dollars per share)		$ 3.89
Warrants contractual term		10 years

Commitments and contingencies - Facilities Leases (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2018 ft²	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Operating lease, term of contract	10 years		10 years
Operation lease, additional premises (in square feet) \| ft²	54,468
Rent expense \| $		$ 1,024	$ 245

Commitments and contingencies - Future Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 2,794
2020	5,757
2021	5,922
2022	6,100
2023	6,283
Thereafter	37,155
Total future minimum lease payment due	$ 64,011

Common stock (Details)		3 Months Ended
Common stock (Details)	Mar. 04, 2019 shares	Mar. 31, 2019 vote $ / shares shares	Dec. 31, 2018 $ / shares shares
Class of Stock [Line Items]
Number of votes per share of common stock \| vote		1
Shares authorized (in shares)		160,000,000
Common shares authorized (in shares)		150,000,000	66,000,000
Common shares par value (in dollars per share) \| $ / shares		$ 0.001	$ 0.001
Preferred shares authorized (in shares)		10,000,000	0
Preferred stock, par value (in dollars per share) \| $ / shares		$ 0.001	$ 0.001
Common Stock
Class of Stock [Line Items]
Conversion of stock, shares converted (in shares)	18,517,386	18,517,386
Conversion Of Convertible Preferred Stock To Common Stock \| Common Stock \| Common Stock
Class of Stock [Line Items]
Conversion of stock, shares converted (in shares)	18,517,386

Stock-based compensation - Narrative (Details) - shares		3 Months Ended
Stock-based compensation - Narrative (Details) - shares	Feb. 27, 2019	Mar. 31, 2019	Mar. 31, 2018	Feb. 26, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)		351,500	0
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued (in shares)				8,395,974
Shares available for future issuance (in shares)				10,558
2019 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued (in shares)	2,168,976
Percent of outstanding shares	4.00%
2019 Employee Stock Purchase Plan \| Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for issuance (in shares)	180,748
Number of additional shares allowable under the plan (in shares)	542,244
Percent of outstanding shares	1.00%

Stock-based compensation - Assumptions to Determine Grant-date Fair Value of Options (Details) - Stock Option	3 Months Ended
	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	66.10%
Expected volatility, maximum	71.10%
Risk-free interest rate, minimum	2.27%
Risk-free interest rate, maximum	2.54%
Expected dividend yield	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 3 months

Stock-based compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Options
Outstanding at beginning of period (in shares)	6,686,267
Granted (in shares)	351,500	0
Exercised (in shares)	(75,313)
Canceled (in shares)	(24,748)
Outstanding at end of period (in shares)	6,937,706		6,686,267
Options exercisable (in shares)	677,605
Options vested or expected to vest (in shares)	6,937,706
Weighted Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 7.50
Granted (in dollars per share)	14.90
Exercised (in dollars per share)	0.75
Canceled (in dollars per share)	7.13
Outstanding at end of period (in dollars per share)	7.95		$ 7.50
Options exercisable (in dollars per share)	2.05
Options vested or expected to vest (in dollars per share)	$ 7.95
Weighted Average Remaining Life and Aggregate Intrinsic Value
Options outstanding ,weighted average remaining life (in years)	9 years		9 years 2 months 12 days
Options exercisable, weighted average remaining life (in years)	7 years 10 months 24 days
Options vested or expected to vest, ,weighted average remaining life (in years)	9 years
Options outstanding, aggregate intrinsic value	$ 47,698		$ 68,167
Options exercisable, aggregate intrinsic value	8,433
Options vested or expected to vest, aggregate intrinsic value	$ 47,698
Fair value of options granted during period (in dollars per share)	$ 9.35
Unrecognized compensation expense	$ 31,575
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense, weighted average remaining term	3 years 1 month 28 days