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Stockholders' Equity - Fair Value of Awards Granted and Vested (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total fair value of awards granted $ 48 $ 37
Total fair value of awards vested 18 15
Restricted Stock Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total fair value of awards granted 30 23
Total fair value of awards vested 16 12
Performance Stock Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total fair value of awards vested 0 3
Performance Stock Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total fair value of awards granted 18 14
Total fair value of awards vested $ 2 $ 0