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Stock-Based Compensation (Details Textuals) - USD ($)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Stock Based Compensation (Textuals) [Abstract]    
Intrinsic value of stock options exercised $ 5,000,000 $ 10,000,000
Total fair value of vested stock options $ 3,000,000 $ 3,000,000
Service-based Restricted Stock Award    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 4 years 4 years
Performance-based Restricted Stock Award    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years 3 years
Employee Stock Option    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recognized compensation expense for accelerated vesting of stock options $ 450,000  
Recognized compensation expense for vesting of stock options 1,000,000  
Unvested share-based payment awards $ 8,000,000  
Employee Service Share Based Compensation Nonvested Awards Compensation Cost Not Yet Recognized Year For Recognition first quarter 2022  
Restricted Stock Award    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recognized compensation expense for accelerated vesting of stock options $ 1,000,000  
Recognized compensation expense for vesting of stock options 3,000,000  
Unvested share-based payment awards $ 29,000,000  
Employee Service Share Based Compensation Nonvested Awards Compensation Cost Not Yet Recognized Year For Recognition first quarter 2022