Retirement Plans and Other Post-Retirement Benefits - Schedule of Net Periodic Costs of Pension and Post Retirement Medical Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Amortization of Deferred Benefit Pension Plan Items [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,504	$ 2,585	$ 5,208	$ 5,796
Interest cost	6,309	5,480	12,480	11,000
Expected return on plan assets	(10,931)	(10,810)	(21,938)	(21,713)
Amortization of prior service cost	921	774	1,655	1,548
Amortization of unrecognized loss	3,014	5,185	5,954	10,474
Net periodic benefit cost	1,817	3,214	3,359	7,105
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	615	789	1,230	1,578
Interest cost	598	545	1,196	1,090
Amortization of prior service cost	(72)	(125)	(144)	(250)
Amortization of unrecognized loss	129	202	258	404
Net periodic benefit cost	$ 1,270	$ 1,411	$ 2,540	$ 2,822