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Equity-Based Compensation - Assumptions Used for Fair Value Measurement (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Forfeiture rate 5.00% 5.00% 5.00% 5.00%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected term 5 years 3 months 18 days 5 years 5 years 5 years 2 months 12 days
Risk-free interest rate 1.97% 1.01% 1.01% 1.59%
Expected volatility rate 80.80% 79.90% 79.20% 78.90%
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected term 6 years 6 months 6 years 10 months 24 days 7 years 7 years
Risk-free interest rate 2.17% 1.41% 2.24% 2.08%
Expected volatility rate 83.10% 82.40% 83.60% 85.40%
Weighted Average        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk-free interest rate 2.07% 1.26% 1.58% 1.73%
Expected volatility rate 81.00% 80.90% 79.90% 82.20%