Fair Value Measurements - Reconciliation of Net Beginning and Ending Balances Recorded for Net Assets and Liabilities Classified as Level 3 (Detail) - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] | ||
| Beginning balance | $ 66 | $ 190 |
| Unrealized and realized losses included in net income | (3) | (12) |
| Settlements of derivative instruments | (3) | (3) |
| Ending balance | $ 60 | $ 175 |