Employee Benefit Plans - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) CompensationPlan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Aggregate deferred under Compensation Plan	$ 2.7	$ 2.4
Number of incentive compensation plans | CompensationPlan	1
Earnings before tax upon achieving a specified earnings threshold	3.00%
Accrued Incentive Compensation expense	$ 9.0	1.6	$ 3.0
Percentage of employee compensation contribution	6.00%
Percentage of vesting contribution	25.00%
Vesting period of employees	5 years
Matching contribution of employees	$ 9.5	10.1	11.8
Safety Incentive Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Compensation expense related to incentive plans	$ 0.6	$ 0.9	$ 0.9