Net Income (Loss) per Share Attributable to Common Stock - Computation of Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income (loss)	$ (15,002)	$ (22,526)	$ (30,236)
Accretion of redeemable convertible preferred stock	(175,915)	(55,502)	(102,224)
Net income (loss) attributable to common stock—basic	(190,917)	(78,028)	(132,460)
Net income (loss) attributable to common stock—diluted	$ (190,917)	$ (78,028)	$ (132,460)
Denominator:
Weighted average shares used to compute net income (loss) per share attributable to common stock—basic and diluted (in shares)	42,722,114	14,395,067	13,416,411
Net income (loss) per share attributable to common stock—basic and diluted (in dollars per share)	$ (4.47)	$ (5.42)	$ (9.87)