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		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Numerator:
Net income (loss)	$ 3,475	$ 10,558	$ (29,756)	$ (13,203)	$ (15,002)
Accretion of preferred stock	0	(40,224)	0	(175,915)
Net income (loss) attributable to common stock—basic	3,475	(29,666)	(29,756)	(189,118)
Net income (loss) attributable to common stock—diluted	$ 3,475	$ (29,666)	$ (29,756)	$ (189,118)
Weighted average shares:
Basic (in shares)	87,743,223	58,868,903	84,327,266	29,678,082
Dilutive shares from stock plans (in shares)	6,899,240	0	0	0
Diluted (in shares)	94,642,463	58,868,903	84,327,266	29,678,082
Net income (loss) per share:
Net income (loss) per share attributable to common stock—basic (in dollars per share)	$ 0.04	$ (0.50)	$ (0.35)	$ (6.37)
Net income (loss) per share attributable to common stock—diluted (in dollars per share)	$ 0.04	$ (0.50)	$ (0.35)	$ (6.37)