Employee Benefit Plans - Components of Net Periodic Defined Benefit Cost (Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 2,072	$ 2,302	$ 2,376
Expected return on plan assets	(2,770)	(2,911)	(3,353)
Amortization of actuarial (gain) loss	1,704	1,474	1,340
Settlement cost	87
Total	1,093	865	363
OPEB
Defined Benefit Plan Disclosure [Line Items]
Interest cost	78	96	108
Amortization of actuarial (gain) loss	(216)	(152)	(101)
Amortization of prior service credit		(541)	(621)
Total	$ (138)	$ (597)	$ (614)