Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) (Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 323	$ 362	$ 645	$ 724
Interest cost on projected benefit obligation	1,238	1,168	2,475	2,336
Net amortization and deferrals	979	663	1,958	1,326
Net periodic benefit cost	$ 582	$ 867	$ 1,162	$ 1,734