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Fair Value Measurements - Fair Value and Carrying Value of Long-Term Debt (Details) - Level 2 - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt (Including Current Maturities) $ 20,733 $ 18,503
Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt (Including Current Maturities) $ 16,506 $ 16,238