Fair Value of Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	$ 825,806	$ 759,551
Equity Securities | United States
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	391,527	357,789
Equity Securities | International
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	82,571	76,189
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	347,612	321,597
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	$ 4,096	$ 3,976