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Stock-Based Compensation (Details) - shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based compensation        
Stock Awards Outstanding (in shares) 8,969,886      
Percentage of stock awards vesting per year 20.00%      
Class A common stock        
Share-based compensation        
Stock Awards Outstanding (in shares) 14,200,000      
Shares of common stock available for future grant under stock incentive plans 60,100,000      
Stock options        
Share-based compensation        
Stock Awards Outstanding (in shares) 14,202,039 8,847,734 7,923,009 6,845,685
Maximum Expiration term 10 years