Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) (Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 362	$ 341	$ 1,086	$ 1,023
Interest cost on projected benefit obligation	1,168	1,447	3,504	4,341
Net amortization and deferrals	663	361	1,989	1,083
Net periodic benefit cost	$ 867	$ 1,427	$ 2,601	$ 4,281