Components of Net Postretirement Costs (Detail) (Other Postretirement Benefit Plans, Defined Benefit, USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the year	$ 464	$ 400	$ 1,392	$ 1,199
Interest cost on accumulated benefit obligation	310	297	931	892
Net amortization of unrecognized amounts, Prior service cost	(104)	(104)	(314)	(312)
Net amortization of unrecognized amounts, Actuarial loss	113	112	339	337
Net Benefit Costs	$ 783	$ 705	$ 2,348	$ 2,116