Postretirement Benefits Other Than Pensions - Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 6,805	$ 6,594	$ 4,729
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service and interest cost	1,725	2,276	2,487
Prior service cost	92	92	65
Unrecognized net (gain) loss	(245)	63	75
Other	0	0	98
Net periodic benefit cost	$ 1,572	$ 2,431	$ 2,725