Pension Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Amount of contributions to contributory plan		$ 45.7	$ 40.7	$ 35.7
Accumulated benefit obligation		110.6	117.7
Fair value of plan assets exceeded accumulated benefit obligation		$ 0.9	$ 10.9
Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Estimated net loss that will be amortized from accumulated other comprehensive income over the next fiscal year	$ 0.9