Convertible Preferred Stock - Issued and Outstanding of convertible perferred stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Summarize the Company’s authorized, issued and outstanding convertible preferred stock
Convertible preferred stock, authorized (in shares)	10,000,000	17,371,402
Convertible preferred stock, issued (in shares)	0	14,500,938	14,500,938
Convertible preferred stock, outstanding (in shares)	0	14,500,938
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, liquidation value	$ 0	$ 195,317
Series A convertible preferred stock
Summarize the Company’s authorized, issued and outstanding convertible preferred stock
Convertible preferred stock, authorized (in shares)		742,409
Convertible preferred stock, outstanding (in shares)		742,409
Net proceeds to date		$ 2,974
Liquidation price (in dollars per share)		$ 4.0751
Convertible preferred stock, liquidation value		$ 3,025
Conversion Price (in dollars per share)		$ 0.0815
Series B convertible preferred stock
Summarize the Company’s authorized, issued and outstanding convertible preferred stock
Convertible preferred stock, authorized (in shares)		455,220
Convertible preferred stock, outstanding (in shares)		455,220
Net proceeds to date		$ 4,939
Liquidation price (in dollars per share)		$ 10.9837
Convertible preferred stock, liquidation value		$ 5,000
Conversion Price (in dollars per share)		$ 0.2197
Series C convertible preferred stock
Summarize the Company’s authorized, issued and outstanding convertible preferred stock
Convertible preferred stock, authorized (in shares)		3,001,448
Convertible preferred stock, outstanding (in shares)		3,001,448
Net proceeds to date		$ 49,824
Liquidation price (in dollars per share)		$ 16.6586
Convertible preferred stock, liquidation value		$ 50,000
Conversion Price (in dollars per share)		$ 3.3317
Series D convertible preferred stock
Summarize the Company’s authorized, issued and outstanding convertible preferred stock
Convertible preferred stock, authorized (in shares)		13,172,325
Convertible preferred stock, outstanding (in shares)		10,301,861
Net proceeds to date		$ 137,132
Liquidation price (in dollars per share)		$ 13.3269
Convertible preferred stock, liquidation value		$ 137,292
Conversion Price (in dollars per share)		$ 13.3269
Convertible preferred stock,
Summarize the Company’s authorized, issued and outstanding convertible preferred stock
Convertible preferred stock, authorized (in shares)		17,371,402
Convertible preferred stock, outstanding (in shares)		14,500,938
Net proceeds to date		$ 194,869
Convertible preferred stock, liquidation value		$ 195,317