Financial Instruments - Assets and Liabilities Not Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Carrying Value [Member] | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Long-term debt, excluding the current portion | $ 32,909 | $ 33,538 |
| Estimated Fair Value [Member] | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Long-term debt, excluding the current portion | 35,260 | 37,253 |
| Level 2 [Member] | Estimated Fair Value [Member] | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Long-term debt, excluding the current portion | $ 35,260 | $ 37,253 |