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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 362	$ 341	$ 724	$ 682
Interest cost on projected benefit obligation	1,168	1,447	2,336	2,894
Net amortization and deferrals	(663)	(361)	(1,326)	(722)
Net periodic benefit cost	$ 867	$ 1,427	$ 1,734	$ 2,854