Fair Value Measurements - Financial Instruments (Details) - USD ($) $ in Millions |
Dec. 31, 2020 |
Dec. 31, 2019 |
|---|---|---|
| Carrying Value | ||
| Financial Instruments | ||
| Long-term debt, excluding current portion - Fair Value | $ 17,989 | $ 17,518 |
| Fair Value | ||
| Financial Instruments | ||
| Long-term debt, excluding current portion - Fair Value | $ 20,496 | $ 18,475 |