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Benefit Obligations - Fair Value of Assets for Non-Qualified Supplemental Pension Plan by Asset Category (Detail) (Level 1 [Member], USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Fair values of the assets for the non-qualified supplemental pension plan by asset category $ 11,462 $ 10,637
Money-market funds [Member]
   
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Fair values of the assets for the non-qualified supplemental pension plan by asset category 1,339 1,353
Mutual Funds [Member]
   
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Fair values of the assets for the non-qualified supplemental pension plan by asset category $ 10,123 $ 9,284