Net Income (Loss) per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income (loss)	$ 1,822	$ (45,074)	$ (27,753)	$ (22,352)	$ (28,090)	$ (17,670)	$ (71,005)	$ (68,112)
Denominator:
Weighted-average common shares outstanding	64,132,283			45,787,137			53,452,007	45,253,176
Less: weighted-average common shares subject to vesting	(1,533,090)			(1,847,856)			(1,599,760)	(1,503,022)
Weighted-average common shares used to compute basic net income (loss) per share	62,599,193			43,939,281			51,852,247	43,750,154
Dilutive effect of stock options	2,538,926
Dilutive effect of ESPP	7,588
Weighted-average common shares used to compute diluted net income (loss) per share	65,145,707			43,939,281			51,852,247	43,750,154
Net income (loss) per share: basic	$ 0.03			$ (0.51)			$ (1.37)	$ (1.56)
Net income (loss) per share: diluted	$ 0.03			$ (0.51)			$ (1.37)	$ (1.56)