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Share-Based Compensation - Assumptions (Details) - Class B common stock issued to management holders [Member] - Stock Option [Member]
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected dividend yield (as a percent) 0.00% 0.00%
Expected volatility (as a percent) 50.00% 66.00%
Risk-free interest rate (as a percent) 2.71% 1.80%
Expected term 6 years 3 months 6 years 2 months