Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) - Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 215	$ 300
Interest cost on projected benefit obligation	787	1,082
Net amortization and deferrals	(1,355)	(711)
Total postretirement benefit expense	$ (353)	$ 671