Redeemable Convertible Preferred Stock and Stockholders' Equity/(Deficit) - Summary of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Temporary Equity And Stockholder's Equity [Abstract]
Redeemable convertible preferred stock outstanding (in shares)	0	55,422,002	55,422,002
Stock options issued and outstanding (in shares)	13,298,339	13,291,684
Shares available for future equity grants (in share)	7,974,192	4,941,504
Total common stock reserved (in shares)	21,272,531	73,655,190