Components of Net Periodic Pension Cost Benefit (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 721	$ 926	$ 1,051
Interest cost	4,465	4,997
Expected return on plan assets	(5,862)	(5,708)	(6,254)
Amortization of actuarial loss	863	3,072	3,164
Net periodic cost	187	3,287	2,873
Components of net periodic cost (benefit)
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 4,465	$ 4,997	$ 4,912