Statements of Convertible Preferred Stock and Shareholders' (Deficit) Equity - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive (Loss) Gain	Accumulated Deficit
Beginning Balance at Dec. 31, 2016	$ (189,734)		$ 1	$ 17,559	$ (82)	$ (207,212)
Beginning Balance, Shares at Dec. 31, 2016			12,154,046
Convertible preferred stock, Beginning Balance at Dec. 31, 2016		$ 511,823
Convertible preferred stock, Beginning Balance, Shares at Dec. 31, 2016		87,797,854
Adjustments to accumulated deficit for adoption of guidance on accounting principle	485					485
Issuance of common stock for cash upon exercise of stock options	$ 95			95
Issuance of common stock for cash upon exercise of stock options, Shares	226,211		54,685
Issuance of Series F-1 convertible preferred stock for cash, net of issuance costs		$ 49,827
Issuance of Series F-1 convertible preferred stock for cash, net of issuance costs, Shares		4,686,649
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value		$ 127
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value, Shares		171,526
Vested Series E-1 convertible preferred stock option forfeitures	$ 644			398		246
Vested Series E-1 convertible preferred stock option forfeitures		$ (644)
Series E-1 convertible preferred stock option share-based compensation	89			89
Adjustment to redemption value for vested Series E-1 convertible preferred stock options	(89)			(89)
Adjustment to redemption value for vested Series E-1 convertible preferred stock options		89
Change in redemption value for vested Series E-1 convertible preferred stock options	(111)					(111)
Change in redemption value for vested Series E-1 convertible preferred stock options		111
Common stock option share-based compensation	6,920			6,920
Other comprehensive income (loss)	(84)				(84)
Net loss	(42,831)					(42,831)
Convertible preferred stock, Ending Balance at Dec. 31, 2017		$ 561,333
Convertible preferred stock, Ending Balance, Shares at Dec. 31, 2017		92,656,029
Ending Balance at Dec. 31, 2017	(224,616)		$ 1	24,972	(166)	(249,423)
Ending Balance, Shares at Dec. 31, 2017			12,208,731
Adjustments to accumulated deficit for adoption of guidance on accounting principle				140		(140)
Issuance of common stock for cash upon exercise of stock options	$ 1,168			1,168
Issuance of common stock for cash upon exercise of stock options, Shares	766,870		632,805
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value		$ 100
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value, Shares		134,065
Vested Series E-1 convertible preferred stock option forfeitures	$ 767			476		291
Vested Series E-1 convertible preferred stock option forfeitures		$ (767)
Series E-1 convertible preferred stock option share-based compensation	3			3
Adjustment to redemption value for vested Series E-1 convertible preferred stock options	(3)			(3)
Adjustment to redemption value for vested Series E-1 convertible preferred stock options		3
Change in redemption value for vested Series E-1 convertible preferred stock options	(189)					(189)
Change in redemption value for vested Series E-1 convertible preferred stock options		189
Common stock option share-based compensation	11,146			11,146
Other comprehensive income (loss)	59				59
Net loss	(46,447)					(46,447)
Convertible preferred stock, Ending Balance at Dec. 31, 2018	$ 560,858	$ 560,858
Convertible preferred stock, Ending Balance, Shares at Dec. 31, 2018	92,790,094	92,790,094
Ending Balance at Dec. 31, 2018	$ (258,112)		$ 1	37,902	(107)	(295,908)
Ending Balance, Shares at Dec. 31, 2018			12,841,536
Proceeds from initial public offering, net of underwriters' discounts and commissions	320,850		$ 2	320,848
Proceeds from initial public offering, net of underwriters' discounts and commissions, Shares			17,250,000
Initial public offering costs	(4,986)			(4,986)
Conversion of convertible preferred stock to common stock	561,931	$ (561,931)	$ 9	561,922
Conversion of convertible preferred stock to common stock, Shares		(93,039,737)	93,039,737
Conversion of convertible preferred stock warrant to common stock warrant	2,602			2,602
Issuance of common stock upon exercise of common stock warrants	9			9
Issuance of common stock upon exercise of common stock warrants, Shares			54,792
Issuance of common stock for cash upon exercise of stock options	$ 4,413			4,413
Issuance of common stock for cash upon exercise of stock options, Shares	2,293,410		2,043,767
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value		$ 109
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value, Shares		249,643
Vesting of restricted stock units			8,310
Change in redemption value for vested Series E-1 convertible preferred stock options	$ (964)					(964)
Change in redemption value for vested Series E-1 convertible preferred stock options		$ 964
Common stock option and restricted stock unit share-based compensation	13,124			13,124
Other comprehensive income (loss)	778				778
Net loss	$ (68,606)					(68,606)
Convertible preferred stock, Ending Balance, Shares at Dec. 31, 2019	0	93,039,737
Ending Balance at Dec. 31, 2019	$ 571,039		$ 12	$ 935,834	$ 671	$ (365,478)
Ending Balance, Shares at Dec. 31, 2019			125,238,142