Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) - Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 214	$ 301	$ 644	$ 901
Interest cost on projected benefit obligation	787	1,082	2,360	3,246
Net amortization and deferrals	(1,355)	(712)	(4,065)	(2,134)
Total postretirement benefit (income) expense	$ (354)	$ 671	$ (1,061)	$ 2,013