Components of Net Postretirement Costs (Detail) (Other Postretirement Benefit Plans, Defined Benefit, USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the year	$ 464	$ 400	$ 928	$ 800
Interest cost on accumulated benefit obligation	310	297	620	595
Net amortization of unrecognized amounts, Prior service cost	(104)	(104)	(209)	(208)
Net amortization of unrecognized amounts, Actuarial loss	113	112	226	224
Net Benefit Costs	$ 783	$ 705	$ 1,565	$ 1,411