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 |