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Investment in Cenovus Energy - Textual (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 5 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]            
Investment in Cenovus Energy $ 1,416   $ 1,416 $ 1,416   $ 1,256
Cenovus Energy Inc [Member]            
Debt and Equity Securities, FV-NI [Line Items]            
(Gain) loss on investment in Cenovus Energy (67) $ 162   (566) $ 1,302  
Common Stock [Member] | Cenovus Energy Inc [Member]            
Debt and Equity Securities, FV-NI [Line Items]            
Investment in equity security, fair value $ 1,400   $ 1,400 $ 1,400    
Shares exchanged for assets held for sale 141   141 141   208
Investment owned, common stock, percent of issued and outstanding 7.00%   7.00% 7.00%    
Cenovus Energy common share price $ 10.06   $ 10.06 $ 10.06    
Equity securities, shares sold 47   67      
Proceeds recognized from sale of equity securities $ 404   $ 584      
Proceeds received from sale of equity securities     $ 569