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Stockholders' Equity - Common Equity (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended 37 Months Ended
Jan. 20, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2020
Jul. 19, 2017
Dec. 19, 2014
Class of Stock [Line Items]              
Common share buy-back program, amount           $ 2,000  
Value of shares repurchased   $ 50 $ 2 $ 273 $ 575    
Common share buy-back program, average price per share         $ 17.71    
Per common share cash dividend declared for the period   $ 1.05 $ 1.00 $ 0.80      
Per common share cash dividend paid in the period   $ 1.0375 $ 0.95 $ 0.725      
Subsequent Event              
Class of Stock [Line Items]              
Per common share cash dividend declared for the period $ 0.2625            
Equity distribution agreement | Class P              
Class of Stock [Line Items]              
Value of Stock Available for Sale Under Equity Distribution Agreement             $ 5,000
Share issued (in shares)   0 0 0      
Common stock              
Class of Stock [Line Items]              
Shares repurchased   4,000,000 100,000 15,000,000 32,000,000