Provision for Federal and Foreign Income Tax Expense for Continuing Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income before income taxes, U.S.	$ 280,535	$ 274,756	$ 259,652
Income before income taxes, Foreign	64,842	85,890	88,015
Income before income taxes	$ 345,377	$ 360,646	$ 347,667