General form of registration statement for all companies including face-amount certificate companies

Stockholders' Equity (Deficit) - Schedule of Stock Option Activity (Detail)

v3.8.0.1
Stockholders' Equity (Deficit) - Schedule of Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options Exercised (3,281)      
2014 Plan Stock Options/Stock Issuance Plan and 2017 Stock Incentive Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options, Outstanding, Beginning Balance 680,434 634,198 588,007  
Options Granted 4,000,508 317,931 236,799  
Options Exercised (3,281) (51,507)    
Options Forfeited / Cancelled (115,719) (220,188) (190,608)  
Options, Outstanding, Ending Balance 4,561,942 680,434 634,198 588,007
Weighted-Average Exercise Price, Outstanding, Beginning Balance $ 2.27 $ 1.50 $ 1.50  
Options, Exercisable at September 30, 2017 1,661,125      
Weighted-Average Exercise Price, Options Granted $ 14.66 3.82 1.50  
Weighted-Average Exercise Price, Options Exercised 1.70 1.50    
Weighted-Average Exercise Price, Options Forfeited / Cancelled 8.56 1.50 1.50  
Weighted-Average Exercise Price, Outstanding, Ending Balance $ 12.99 $ 2.27 $ 1.50 $ 1.50
Weighted-Average Remaining Contractual Term, Outstanding 8 years 11 months 23 days 8 years 5 months 27 days 8 years 10 months 3 days 9 years 8 months 5 days
Weighted-Average Aggregate Intrinsic Value $ 155,197 $ 3,031