Employee Benefit Plans - Key Actuarial Assumptions Used to Determine Net Benefit Obligation (Detail) - OPEB |
12 Months Ended | |
|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Pension Plans Postretirement And Other Employee Benefits [Line Items] | ||
| Initial rate | 7.00% | 7.00% |
| Ultimate rate | 5.00% | 5.00% |
| Year of ultimate rate achievement | 2021 | 2021 |
| Weighted average discount rate | 3.50% | 3.90% |