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Share-based Compensation (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 57.30%    
Weighted average volatility 57.30% 58.44% 61.29%
Expected dividends 0.00% 0.00% 0.00%
Risk-free rate 3.10%    
Contractual term (in years) 10 years 10 years 10 years
Enterprise value $ 0.65    
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility   57.90% 59.60%
Risk-free rate   2.60% 1.82%
Enterprise value   $ 0.32 $ 0.30
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility   59.70% 62.40%
Risk-free rate   3.18% 2.70%
Enterprise value   $ 0.65 $ 0.32