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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,850	$ 2,602	$ 8,546	$ 7,810
Interest cost	5,868	6,216	17,606	18,696
Expected return on plan assets	(11,498)	(10,969)	(34,495)	(32,907)
Amortization of prior service cost	760	827	2,281	2,482
Amortization of unrecognized loss	4,226	2,977	12,679	8,931
Net periodic benefit cost	2,206	1,653	6,617	5,012
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	358	614	1,074	1,844
Interest cost	499	597	1,498	1,793
Amortization of prior service cost	(70)	(72)	(210)	(216)
Amortization of unrecognized loss	58	129	173	387
Net periodic benefit cost	$ 845	$ 1,268	$ 2,535	$ 3,808