Employee Retirement Plans - Defined Benefit Plans - Additional Information (Detail) - USD ($) |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2016 |
Jun. 30, 2015 |
Jun. 30, 2016 |
Jun. 30, 2015 |
|
| Defined Benefit Plan Disclosure [Line Items] | ||||
| Employer contribution | $ 0 | $ 0 | $ 0 | $ 0 |
| Pension Benefits [Member] | ||||
| Defined Benefit Plan Disclosure [Line Items] | ||||
| Estimated payments for the current year | 2,100,000 | 2,100,000 | ||
| OPEB Benefits [Member] | ||||
| Defined Benefit Plan Disclosure [Line Items] | ||||
| Changes in OPEB benefit obligation | 700,000 | |||
| Amount recognized in other comprehensive income | 700,000 | |||
| Estimated payments for the current year | $ 7,300,000 | $ 7,300,000 | ||