Postretirement Benefits Other Than Pensions - Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 6,717	$ 6,805	$ 6,594
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service and interest cost	1,545	1,725	2,276
Prior service cost	92	92	92
Unrecognized net (gain) loss	(457)	(245)	63
Net periodic benefit cost	$ 1,180	$ 1,572	$ 2,431