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