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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 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2,695 $ 2,614 $ 8,065 $ 7,855
Interest cost 5,929 6,120 17,825 18,360
Expected return on plan assets (10,749) (11,331) (32,246) (33,992)
Amortization of prior service cost/(credit) 708 674 2,120 2,023
Amortization of unrecognized/actuarial loss (gain) 3,078 3,293 9,211 9,880
Total net periodic benefit cost 1,661 1,370 4,975 4,126
Other Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 289 287 868 860
Interest cost 499 498 1,497 1,494
Amortization of prior service cost/(credit) (45) (45) (136) (136)
Amortization of unrecognized/actuarial loss (gain) (94) (189) (283) (567)
Total net periodic benefit cost $ 649 $ 551 $ 1,946 $ 1,651