Employee Benefit Plans - Postretirement Benefits Other Than Pensions (OPEB) - Additional Information (Detail) - OPEB - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Defined Benefit Plan Disclosure [Line Items] | |||
| Health care benefits | 65 years | ||
| Unrecognized actuarial losses to be recognized in next fiscal year | $ 0.2 | ||
| Unrecognized prior service credit to be recognized in next fiscal year | 0.6 | ||
| Accumulated benefit obligations (ABO) | $ 8.1 | $ 7.2 | $ 7.0 |
| Discount rate | 3.50% | 3.90% | 3.70% |
| United States | |||
| Defined Benefit Plan Disclosure [Line Items] | |||
| Accumulated benefit obligations (ABO) | $ 0.3 | $ 0.5 | |
| Canada | |||
| Defined Benefit Plan Disclosure [Line Items] | |||
| Accumulated benefit obligations (ABO) | $ 7.8 | $ 6.7 | |