Benefit Plans - Qualified and Non-Qualified Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income or Loss (Detail) - Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net loss (gain)	$ 3,843	$ (5,837)
Prior service cost	0	462
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	(35)	0
Amortization of actuarial loss	(381)	(294)
Total recognized in other comprehensive income	3,427	(5,669)
Total recognized in net periodic pension income and other comprehensive income/loss (before tax)	$ (5,607)	$ (13,233)