Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator
Net loss									$ (276,213)	$ (67,698)	$ (57,010)
Denominator
Weighted average shares outstanding									44,767,581	38,466,233	33,098,571
Less: weighted average shares subject to repurchase									(2,085)	(79,327)	(266,057)
Weighted average shares used to compute basic and diluted net loss per share	46,278,409	46,133,068	46,065,901	40,506,313	40,259,575	40,116,644	37,440,024	35,827,235	44,765,496	38,386,906	32,832,514
Net loss per share, basic and diluted	$ (1.27)	$ (3.07)	$ (0.83)	$ (0.93)	$ (0.39)	$ (0.39)	$ (0.49)	$ (0.50)	$ (6.17)	$ (1.76)	$ (1.74)