Employee Benefit Plans - Summary of Actuarial Assumptions Used to Determine Net Benefit Obligation (Detail) - OPEB |
12 Months Ended | |
|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Defined Benefit Plan Disclosure [Line Items] | ||
| Initial rate | 6.50% | 7.00% |
| Ultimate rate | 4.50% | 5.00% |
| Year of ultimate rate achievement | 2021 | 2021 |
| Discount rate | 3.10% | 3.20% |