Pension Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Amount of contributions to contributory plan	$ 50,100,000	$ 45,700,000	$ 40,700,000
Accumulated benefit obligation	115,900,000	110,600,000
Fair value of plan assets exceeded accumulated benefit obligation	$ 2,700,000
Fair value of plan accumulated benefit obligation exceeds plan assets		$ 900,000
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
Estimated prior service cost that will be amortized from accumulated other comprehensive income over the next fiscal year				$ 0