Retirement Benefits - Fair Value of Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	$ 732,502	$ 825,806
Equity Securities | United States
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	350,728	391,527
Equity Securities | International
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	73,415	82,571
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	304,128	347,612
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	$ 4,231	$ 4,096