Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) - Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 214	$ 301	$ 429	$ 601
Interest cost on projected benefit obligation	787	1,082	1,574	2,164
Net amortization and deferrals	(1,355)	(712)	(2,710)	(1,423)
Total postretirement benefit (income) expense	$ (354)	$ 671	$ (707)	$ 1,342