Postretirement Benefits Other Than Pensions - Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 6,594	$ 4,729	$ 10,400
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service and interest cost	2,276	2,487	2,039
Prior service cost	92	65	26
Unrecognized net loss	63	75	298
Other	0	98	2,286
Net periodic benefit cost	$ 2,431	$ 2,725	$ 4,649