Condensed Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity/(Deficit) Statement - USD ($)	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Redeemable convertible preferred stock beginning balance (in shares) at Dec. 31, 2015	55,422,002
Redeemable convertible preferred stock beginning balance at Dec. 31, 2015	$ 599,915,000
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	$ 55,501,000
Redeemable convertible preferred stock ending balance (in shares) at Dec. 31, 2016	55,422,002
Redeemable convertible preferred stock ending balance at Dec. 31, 2016	$ 655,416,000
Common stock, outstanding, beginning balance (in shares) at Dec. 31, 2015		14,059,601
Total stockholders' deficit, beginning balance at Dec. 31, 2015	(495,713,000)	$ 14,000	$ 0	$ (495,727,000)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)		627,423
Exercise of stock options	1,495,000	$ 1,000	1,494,000
Stock-based compensation	8,512,000		8,512,000
Accretion of redeemable convertible preferred stock	(55,502,000)		(10,006,000)	(45,496,000)
Net income (loss)	$ (22,526,000)			(22,526,000)
Common stock, outstanding, ending balance (in shares) at Dec. 31, 2016	14,687,024	14,687,024
Total stockholders' deficit, ending balance at Dec. 31, 2016	$ (563,734,000)	$ 15,000	0	(563,749,000)
Increase (Decrease) in Stockholders' Equity
Cumulative stock-based compensation adjustment (see Note 1)	0		522,000	(522,000)
Accretion of redeemable convertible preferred stock	$ 175,915,000
Conversion of redeemable convertible preferred stock to common stock (in shares)	(55,422,002)
Conversion of redeemable convertible preferred stock to common stock	$ (831,331,000)
Redeemable convertible preferred stock ending balance (in shares) at Sep. 30, 2017	0
Redeemable convertible preferred stock ending balance at Sep. 30, 2017	$ 0
Increase (Decrease) in Stockholders' Equity
Proceeds from initial public offering, net of underwriters' discounts (in shares)		10,615,650.00
Proceeds from initial public offering, net of underwriters' discounts of $11,146	148,088,000	$ 10,000	148,078,000
Underwriters' discounts	11,146,000
Initial public offering costs	$ (3,799,000)		(3,799,000)
Exercise of stock options (in shares)	660,634	660,634
Exercise of stock options	$ 2,519,000	$ 1,000	2,518,000
Stock-based compensation	8,222,000		8,222,000
Accretion of redeemable convertible preferred stock	(175,915,000)		(8,690,000)	(167,225,000)
Conversion of redeemable convertible preferred stock to common stock (in shares)		55,422,002
Conversion of redeemable convertible preferred stock to common stock	831,330,000	$ 55,000	213,780,000	617,494,842.000
Net income (loss)	$ (13,203,000)
Common stock, outstanding, ending balance (in shares) at Sep. 30, 2017	81,385,310	81,385,310
Total stockholders' deficit, ending balance at Sep. 30, 2017	$ 233,508,000	$ 81,000	$ 360,631,000	$ (127,204,000)