Fair Value Measurements - Financial Instruments (Details) - USD ($) $ in Millions |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Carrying Value | ||
| Financial Instruments | ||
| Long-term debt, excluding current portion - Fair Value | $ 13,411 | $ 12,096 |
| Fair Value | ||
| Financial Instruments | ||
| Long-term debt, excluding current portion - Fair Value | $ 13,586 | $ 12,535 |