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Fair Value of Debt (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure $ 731 $ 927
Reported Value Measurement [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 37,324 37,843
Estimate of Fair Value Measurement [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure $ 37,469 $ 40,050