Postretirement Benefits Other Than Pension Expenses (Detail) (Postretirement Benefits Plan [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 341	$ 368	$ 1,023	$ 884
Interest cost on projected benefit obligation	1,447	1,487	4,341	4,396
Net amortization and deferrals	(361)	(571)	(1,083)	(1,914)
Total postretirement benefit expense	$ 1,427	$ 1,284	$ 4,281	$ 3,366