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Components of Net Postretirement Costs (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Pension Plans
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 22,424 $ 19,808 $ 18,315
Interest cost 14,800 13,473 12,091
Expected return on plan assets (12,108) (13,544) (11,207)
Net amortization of unrecognized amounts, Prior service cost 5,993 5,782 5,685
Net amortization of unrecognized amounts, Actuarial loss 4,916 411 247
Net periodic benefit cost 36,025 25,930 25,131
Postretirement Plans
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 1,856 1,599 1,399
Interest cost 1,241 1,189 1,130
Net amortization of unrecognized amounts, Prior service cost (419) (416) (416)
Net amortization of unrecognized amounts, Actuarial loss 452 449 342
Net periodic benefit cost $ 3,130 $ 2,821 $ 2,455