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Components of Net Postretirement Costs (Detail) (Postretirement Plans, USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Postretirement Plans
   
Defined Benefit Plan Disclosure [Line Items]    
Service cost for benefits earned during the year $ 515 $ 464
Interest cost on accumulated benefit obligation 311 310
Net amortization of unrecognized amounts, Prior service cost (106) (104)
Net amortization of unrecognized amounts, Actuarial loss 134 113
Net Benefit Costs $ 854 $ 783