Condensed Statements of Convertible Preferred Stock and Shareholders' Deficit - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive (loss) Income	Accumulated Deficit
Beginning Balance at Dec. 31, 2017	$ (224,616)		$ 1	$ 24,972	$ (166)	$ (249,423)
Beginning Balance, Shares at Dec. 31, 2017			12,208,731
Convertible preferred stock, Beginning Balance at Dec. 31, 2017		$ 561,333
Convertible preferred stock, Beginning Balance, Shares at Dec. 31, 2017		92,656,029
Adjustments to accumulated deficit for adoption of guidance on accounting for share-based payment transactions (unaudited)				140		(140)
Issuance of common stock for cash upon exercise of stock options (unaudited)	823			823
Issuance of common stock for cash upon exercise of stock options (unaudited), Shares			338,113
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value (unaudited)		$ 100
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value (unaudited), Shares		134,065
Vested Series E-1 convertible preferred stock option forfeitures (unaudited)	767			476		291
Vested Series E-1 convertible preferred stock option forfeitures (unaudited)		$ (767)
Series E-1 convertible preferred stock option share-based compensation (unaudited)	3			3
Adjustment to redemption value for vested Series E-1 convertible preferred stock options (unaudited)	(3)			(3)
Adjustment to redemption value for vested Series E-1 convertible preferred stock options (unaudited)		3
Change in redemption value for vested Series E-1 convertible preferred stock options (unaudited)	2					2
Change in redemption value for vested Series E-1 convertible preferred stock options (unaudited)		(2)
Common stock option share-based compensation (unaudited)	5,547			5,547
Other comprehensive income (unaudited)	(74)				(74)
Net loss	(24,884)					(24,884)
Convertible preferred stock, Ending Balance at Jun. 30, 2018		$ 560,667
Convertible preferred stock, Ending Balance, Shares at Jun. 30, 2018		92,790,094
Ending Balance at Jun. 30, 2018	(242,435)		$ 1	31,958	(240)	(274,154)
Ending Balance, Shares at Jun. 30, 2018			12,546,844
Beginning Balance at Mar. 31, 2018	(233,612)		$ 1	28,620	(283)	(261,950)
Beginning Balance, Shares at Mar. 31, 2018			12,301,844
Convertible preferred stock, Beginning Balance at Mar. 31, 2018		$ 561,396
Convertible preferred stock, Beginning Balance, Shares at Mar. 31, 2018		92,745,734
Issuance of common stock for cash upon exercise of stock options (unaudited)	415			415
Issuance of common stock for cash upon exercise of stock options (unaudited), Shares			245,000
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value (unaudited)		$ 35
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value (unaudited), Shares		44,360
Vested Series E-1 convertible preferred stock option forfeitures (unaudited)	767			476		291
Vested Series E-1 convertible preferred stock option forfeitures (unaudited)		$ (767)
Series E-1 convertible preferred stock option share-based compensation (unaudited)	1			1
Adjustment to redemption value for vested Series E-1 convertible preferred stock options (unaudited)	(1)			(1)
Adjustment to redemption value for vested Series E-1 convertible preferred stock options (unaudited)		1
Change in redemption value for vested Series E-1 convertible preferred stock options (unaudited)	(2)					(2)
Change in redemption value for vested Series E-1 convertible preferred stock options (unaudited)		2
Common stock option share-based compensation (unaudited)	2,447			2,447
Other comprehensive income (unaudited)	43				43
Net loss	(12,493)					(12,493)
Convertible preferred stock, Ending Balance at Jun. 30, 2018		$ 560,667
Convertible preferred stock, Ending Balance, Shares at Jun. 30, 2018		92,790,094
Ending Balance at Jun. 30, 2018	(242,435)		$ 1	31,958	(240)	(274,154)
Ending Balance, Shares at Jun. 30, 2018			12,546,844
Beginning Balance at Dec. 31, 2018	(258,112)		$ 1	37,902	(107)	(295,908)
Beginning Balance, Shares at Dec. 31, 2018			12,841,536
Convertible preferred stock, Beginning Balance at Dec. 31, 2018	$ 560,858	$ 560,858
Convertible preferred stock, Beginning Balance, Shares at Dec. 31, 2018	92,790,094	92,790,094
Issuance of common stock for cash upon exercise of stock options (unaudited)	$ 1,880			1,880
Issuance of common stock for cash upon exercise of stock options (unaudited), Shares			883,845
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value (unaudited)		$ 109
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value (unaudited), Shares		249,643
Change in redemption value for vested Series E-1 convertible preferred stock options (unaudited)	(964)					(964)
Change in redemption value for vested Series E-1 convertible preferred stock options (unaudited)		$ 964
Common stock option share-based compensation (unaudited)	6,378			6,378
Other comprehensive income (unaudited)	489				489
Net loss	(34,045)					(34,045)
Convertible preferred stock, Ending Balance at Jun. 30, 2019	$ 561,931	$ 561,931
Convertible preferred stock, Ending Balance, Shares at Jun. 30, 2019	93,039,737	93,039,737
Ending Balance at Jun. 30, 2019	$ (284,374)		$ 1	46,160	382	(330,917)
Ending Balance, Shares at Jun. 30, 2019			13,725,381
Beginning Balance at Mar. 31, 2019	(273,474)		$ 1	40,981	92	(314,548)
Beginning Balance, Shares at Mar. 31, 2019			12,930,536
Convertible preferred stock, Beginning Balance at Mar. 31, 2019		$ 561,210
Convertible preferred stock, Beginning Balance, Shares at Mar. 31, 2019		93,023,694
Issuance of common stock for cash upon exercise of stock options (unaudited)	1,847			1,847
Issuance of common stock for cash upon exercise of stock options (unaudited), Shares			794,845
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value (unaudited)		$ 11
Issuance of Series E-1 convertible preferred stock for cash upon exercise of Series E-1 convertible preferred stock options at fair value (unaudited), Shares		16,043
Change in redemption value for vested Series E-1 convertible preferred stock options (unaudited)	(710)					(710)
Change in redemption value for vested Series E-1 convertible preferred stock options (unaudited)		$ 710
Common stock option share-based compensation (unaudited)	3,332			3,332
Other comprehensive income (unaudited)	290				290
Net loss	(15,659)					(15,659)
Convertible preferred stock, Ending Balance at Jun. 30, 2019	$ 561,931	$ 561,931
Convertible preferred stock, Ending Balance, Shares at Jun. 30, 2019	93,039,737	93,039,737
Ending Balance at Jun. 30, 2019	$ (284,374)		$ 1	$ 46,160	$ 382	$ (330,917)
Ending Balance, Shares at Jun. 30, 2019			13,725,381