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Stockholders' Equity - Fair Value of Awards Granted and Vested (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of restricted shares granted $ 67 $ 53 $ 44
Fair value of restricted shares vested 23 21 18
RSAs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of restricted shares granted 44 33 28
Fair value of restricted shares vested 22 19 15
PSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of restricted shares granted 23 20 16
Fair value of restricted shares vested 1 2 0
PSAs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of restricted shares vested $ 0 $ 0 $ 3