Employee Benefits - Components of Net Periodic Benefit Cost and Changes in Plan Assets and Benefit Obligations Recognized in OCI (Detail) - Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 852	$ 966	$ 1,295
Interest cost on projected benefit obligation	3,326	3,904	4,896
Net amortization of deferrals	(5,088)	(4,476)	(4,109)
Net periodic benefit cost	(910)	394	$ 2,082
Net actuarial loss (gain)	2,868	(1,557)
Recognized actuarial loss	(1,701)	(1,331)
Prior service credit	0	(33,902)
Recognized prior service credit	6,789	5,807
Total recognized in OCI (before tax effects)	$ 7,956	$ (30,983)