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Fair Value Measurements - Reported (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 20,455 $ 19,800
SMR liability 76 80
Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt 21,721 21,054
SMR liability $ 87 $ 90