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Retirement Plans and Other Post-Retirement Benefits - Schedule of Net Periodic Costs of Pension and Post Retirement Medical Benefit Plans (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Amortization of Deferred Benefit Pension Plan Items [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 2,703 $ 3,211
Interest cost 6,171 5,520
Expected return on plan assets (11,007) (10,903)
Amortization of prior service cost 734 774
Amortization of unrecognized loss 2,940 5,289
Net periodic benefit cost 1,541 3,891
Other Benefits [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 615 789
Interest cost 598 545
Amortization of prior service cost (72) (125)
Amortization of unrecognized loss 129 202
Net periodic benefit cost $ 1,270 $ 1,411