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Stock-Based Compensation (Details Textuals) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Stock Based Compensation (Textuals) [Abstract]    
Intrinsic value of stock options exercised $ 9 $ 11
Total fair value of vested stock options $ 4 $ 4
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 25.00%  
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 150.00%  
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
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
Employee Stock Option    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Recognized compensation expense for vesting of stock options $ 2  
String_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost less than $1 million  
Unvested share-based payment awards $ 7  
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 vesting of stock options $ 6 $ 11
Recognized compensation expense for accelerated vesting of stock options 1  
Unvested share-based payment awards $ 25  
Employee Service Share Based Compensation Nonvested Awards Compensation Cost Not Yet Recognized Year For Recognition first quarter 2022