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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Numerator:
Net income (loss)	$ 10,558	$ 5,700	$ (13,203)	$ (17,191)	$ (22,526)
Accretion of redeemable convertible preferred stock	(40,224)	(3,050)	(175,915)	56,819
Undistributed earnings attributable to participating securities	0	(2,105)	0	(31,483)
Net income (loss) attributable to common stock—basic	(29,666)	545	(189,118)	8,145
Net income (loss) attributable to common stock—diluted	$ (29,666)	$ 545	$ (189,118)	$ (17,191)
Denominator:
Weighted average shares used to compute net income (loss) per share attributable to common stock—basic	58,868,903	14,441,246	29,678,082	14,339,820
Weighted average shares used to compute net income (loss) per share attributable to common stock—diluted	58,868,903	17,855,205	29,678,082	69,761,822
Net income (loss) per share attributable to common stock - basic (in dollars per share)	$ (0.50)	$ 0.04	$ (6.37)	$ 0.57
Net income (loss) per share attributable to common stock - diluted (in dollars per share)	$ (0.50)	$ 0.03	$ (6.37)	$ (0.25)
Net income (loss) attributable to common stock—basic	$ (29,666)	$ 545	$ (189,118)	$ 8,145
Add-back: Accretion due to application of if-converted	40,224	3,050	175,915	(56,819)
Add-back: Undistributed earnings attributable to participating securities due to application of if-converted	0	2,105	0	31,483
Net income (loss) attributable to common stock—diluted	$ (29,666)	$ 545	$ (189,118)	$ (17,191)