Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) (Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 362	$ 341
Interest cost on projected benefit obligation	1,168	1,447
Net amortization and deferrals	(663)	(361)
Total postretirement benefit expense	$ 867	$ 1,427