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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,510	$ 2,561	$ 5,240	$ 5,696
Interest cost	6,153	5,788	12,240	11,738
Expected return on plan assets	(11,275)	(11,454)	(22,661)	(22,997)
Amortization of prior service cost	675	761	1,349	1,521
Amortization of unrecognized (gain)/loss	3,533	3,947	6,587	8,453
Net periodic benefit cost	1,596	1,603	2,755	4,411
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	250,000	303,000	573,000	716,000
Interest cost	456,000	436,000	996,000	999,000
Amortization of prior service cost	(46,000)	(70,000)	(91,000)	(140,000)
Amortization of unrecognized (gain)/loss	(327,000)	15,000	(378,000)	115,000
Net periodic benefit cost	$ 333,000	$ 684,000	$ 1,100,000	$ 1,690,000