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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,649	$ 2,510	$ 5,370	$ 5,240
Interest cost	5,989	6,153	11,896	12,240
Expected return on plan assets	(10,666)	(11,275)	(21,497)	(22,661)
Amortization of prior service cost/(credit)	708	675	1,412	1,349
Amortization of unrecognized/actuarial loss (gain)	3,311	3,533	6,133	6,587
Total net periodic benefit cost	1,991	1,596	3,314	2,755
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	284	250	579	573
Interest cost	513	456	998	996
Amortization of prior service cost/(credit)	(46)	(46)	(91)	(91)
Amortization of unrecognized/actuarial loss (gain)	(46)	(327)	(189)	(378)
Total net periodic benefit cost	$ 705	$ 333	$ 1,297	$ 1,100