CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock
Beginning balance (in shares) at Dec. 31, 2016						15,848	39,538
Beginning balance at Dec. 31, 2016						$ 49,913	$ 227,059
Redeemable Convertible Preferred Stock [Roll Forward]
Accretion of Series A and B redeemable convertible preferred stock						$ 22	$ 741
Ending balance (in shares) at Dec. 31, 2017						15,848	39,538
Ending balance at Dec. 31, 2017						$ 49,935	$ 227,800
Beginning balance (in shares) at Dec. 31, 2016		21,165
Beginning balance at Dec. 31, 2016	$ (301,918)	$ 212	$ 11,017	$ 0	$ (313,147)
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Accretion of Series A and B redeemable convertible preferred stock	$ (763)		(763)
Issuance of restricted stock award (in shares)	1,583
Issuance of restricted stock award	$ 0	$ 16	(16)
Exercise of stock options (in shares)	1,870	1,870
Exercise of stock options	$ 3,020	$ 19	3,001
Repurchase of common stock (in shares)		(146)
Repurchase of common stock	(385)	$ (1)	(384)
Stock-based compensation	7,760		7,760
Other comprehensive income	0
Net loss	(41,022)				(41,022)
Ending balance (in shares) at Dec. 31, 2017		24,472
Ending balance at Dec. 31, 2017	(371,665)	$ 246	20,676	0	(392,587)
Redeemable Convertible Preferred Stock [Roll Forward]
Accretion of Series A and B redeemable convertible preferred stock						$ 13	$ 421
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)						(15,848)	(39,538)
Conversion of redeemable convertible preferred stock to common stock upon initial public offering						$ (49,948)	$ (228,221)
Ending balance (in shares) at Dec. 31, 2018						0	0
Ending balance at Dec. 31, 2018						$ 0	$ 0
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Accretion of Series A and B redeemable convertible preferred stock	$ (434)		(434)
Exercise of stock options (in shares)	740	740
Exercise of stock options	$ 1,668	$ 7	1,661
Repurchase of common stock (in shares)		(7)
Repurchase of common stock	(75)	$ (1)	(74)
Issuance of common stock in connection with initial public offering, net of underwriting discounts and commissions and other offering expenses (in shares)		12,535
Issuance of common stock in connection with initial public offering, net of underwriting discounts and commissions and other offering expenses	264,599	$ 125	264,474
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)		55,386
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	278,169	$ 554	277,615
Stock-based compensation	23,022		23,022
Other comprehensive income	0
Net loss	$ (73,521)				(73,521)
Ending balance (in shares) at Dec. 31, 2018	93,126	93,126
Ending balance at Dec. 31, 2018	$ 121,763	$ 931	586,940	0	(466,108)
Ending balance (in shares) at Dec. 31, 2019						0	0
Ending balance at Dec. 31, 2019						$ 0	$ 0
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (in shares)	4,205	4,205
Exercise of stock options	$ 19,048	$ 42	19,006
Vesting of restricted stock units (in shares)		479
Vesting of restricted stock units	0	$ 5	(5)
Issuance of common stock under employee stock purchase plan (in shares)		777
Issuance of common stock under employee stock purchase plan	15,129	$ 8	15,121
Stock-based compensation	41,928		41,928
Other comprehensive income	50			50
Net loss	$ (99,013)				(99,013)
Ending balance (in shares) at Dec. 31, 2019	98,587	98,587
Ending balance at Dec. 31, 2019	$ 98,905	$ 986	$ 662,990	$ 50	$ (565,121)