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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 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Amortization of Deferred Benefit Pension Plan Items [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2,602 $ 2,897 $ 7,810 $ 8,693
Interest cost 6,216 5,504 18,696 16,504
Expected return on plan assets (10,969) (10,857) (32,907) (32,570)
Amortization of prior service cost 827 773 2,482 2,321
Amortization of unrecognized loss 2,977 5,237 8,931 15,711
Net periodic benefit cost 1,653 3,554 5,012 10,659
Other Benefits [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 614 789 1,844 2,367
Interest cost 597 545 1,793 1,635
Amortization of prior service cost (72) (125) (216) (375)
Amortization of unrecognized loss 129 202 387 606
Net periodic benefit cost $ 1,268 $ 1,411 $ 3,808 $ 4,233