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Fair Value Measurements (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Reported Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt, fair value $ (12,681) $ (12,196)
Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt, fair value $ (16,664) $ (14,806)