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

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

v3.8.0.1
Stockholders' Equity (Deficit) - Schedule of Restricted Stock Activity (Detail) - Restricted Stock [Member] - $ / shares
 9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares, Forfeited (22,018)    
2014 Plan Stock Options/Stock Issuance Plan and 2017 Stock Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares, Unvested, Beginning Balance 208,886 79,650 133,569
Shares, Granted 52,433 213,940 108,345
Shares, Forfeited (22,018) (14,143) (120,696)
Shares, Vested (78,399) (70,561) (41,568)
Shares, Unvested, Ending Balance 160,902 208,886 79,650
Weighted Average Grant Date Fair Value, Unvested, Beginning Balance $ 4.80 $ 1.50 $ 1.50
Weighted Average Grant Date Fair Value, Granted 14.79 4.75 1.42
Weighted Average Grant Date Fair Value, Forfeited 8.43 3.88 1.43
Weighted Average Grant Date Fair Value, Vested 7.82 1.50 1.50
Weighted Average Grant Date Fair Value, Unvested, Ending Balance $ 6.08 $ 4.80 $ 1.50