Employee Benefit Plans - Components of Net Periodic Benefit Cost (Credit) (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Pension Plans And Defined Benefit Postretirement Plans Disclosure [Abstract]
Tax expense (benefit) on other changes in plan assets and benefit obligations recognized in AOCI	$ (1,287)	$ (3,448)	$ 7,151