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Share-Based Compensation - Assumptions (Details) - Employee Stock Option [Member]
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 2 years 3 months    
Class B common stock issued to management holders [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend Yield 0.00% 0.00% 0.00%
Risk free interest rate 2.71% 1.80% 1.64%
Expected term 6 years 3 months 15 days 6 years 2 months 16 days 6 years 4 months
Expected volatility 50.00% 66.00% 56.00%