Redeemable Convertible Preferred Stock and Stockholders' Equity/(Deficit) - Narrative (Details)	Sep. 30, 2017 $ / shares shares	Aug. 02, 2017	Dec. 31, 2016 $ / shares shares	Dec. 31, 2015 shares
Temporary Equity And Stockholder's Equity [Abstract]
Conversion basis		1
Redeemable convertible preferred stock, authorized (in shares)	0		166,266,114
Redeemable convertible preferred stock, issued (in shares)	0		55,422,002
Redeemable convertible preferred stock outstanding (in shares)	0		55,422,002	55,422,002
Common stock, authorized (in shares)	500,000,000		290,081,638
Common stock, par value (in dollars per share) | $ / shares	$ 0.001		$ 0.001
Preferred stock, authorized (in shares)	10,000,000		0
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001		$ 0.001
Preferred stock, outstanding (in shares)	0		0