Net Income (loss) per Share Attributable to Common Stock - Computation of Net Income (loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Numerator:
Net income (loss)	$ 3,207	$ 4,304	$ (33,232)	$ (23,762)	$ (15,002)
Accretion of preferred stock	0	(110,921)	0	(135,690)
Net income (loss) attributable to common stock—basic	3,207	(106,617)	(33,232)	(159,452)
Net income (loss) attributable to common stock—diluted	$ 3,207	$ (106,617)	$ (33,232)	$ (159,452)
Weighted average shares:
Basic (in shares)	83,164,670	14,913,234	82,590,979	14,840,759
DIlutive shares from stock plans (in shares)	7,578,508	0	0	0
Diluted (in shares)	90,743,178	14,913,234	82,590,979	14,840,759
Net income (loss) per share:
Net income (loss) per share attributable to common stock—basic (in dollars per share)	$ 0.04	$ (7.15)	$ (0.40)	$ (10.74)
Net income (loss) per share attributable to common stock—diluted (in dollars per share)	$ 0.04	$ (7.15)	$ (0.40)	$ (10.74)