Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) (Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 323	$ 362
Interest cost on projected benefit obligation	1,238	1,168
Net amortization and deferrals	978	663
Net periodic benefit cost	$ 583	$ 867