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Stock-Based Compensation (Details) - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Stock Based Compensation (Textuals) [Abstract]      
Intrinsic value of stock options exercised $ 10 $ 13 $ 9
Total fair value of vested stock options $ 4 4 3
2017 Incentive Compensation Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting Period 4 years    
Expiration term of awards 10 years    
Shares remaining available for grant (in shares) 11.6    
Performance-based Restricted Stock Award [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting Period 3 years    
Term of transfer restrictions 3 years    
Service-based Restricted Stock Award [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting Period 4 years    
Term of transfer restrictions 4 years    
Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Recognized compensation expense for vesting of stock options $ 4 4 4
String_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost less than $1 million    
Unvested share-based payment awards $ 4    
Employee Service Share Based Compensation Nonvested Awards Compensation Cost Not Yet Recognized Year For Recognition first quarter 2022    
Restricted Stock Award [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Recognized compensation expense for accelerated vesting of stock options $ 1    
Recognized compensation expense for vesting of stock options 13 $ 18 $ 18
Unvested share-based payment awards $ 18    
Employee Service Share Based Compensation Nonvested Awards Compensation Cost Not Yet Recognized Year For Recognition first quarter 2022    
Maximum      
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%    
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%