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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
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 2,730 $ 3,136
Interest cost 6,087 5,950
Expected return on plan assets (11,386) (11,543)
Amortization of prior service cost 674 760
Amortization of unrecognized (gain)/loss 3,054 4,506
Net periodic benefit cost 1,159 2,809
Other Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 323,000 413,000
Interest cost 540,000 563,000
Amortization of prior service cost (45,000) (70,000)
Amortization of unrecognized (gain)/loss (51,000) 100,000
Net periodic benefit cost $ 767,000 $ 1,006,000