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
	Mar. 31, 2015	Mar. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,136	$ 2,703
Interest cost	5,950	6,171
Expected return on plan assets	(11,543)	(11,007)
Amortization of prior service cost	760	734
Amortization of unrecognized loss	4,506	2,940
Net periodic benefit cost	2,809	1,541
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	413	615
Interest cost	563	598
Amortization of prior service cost	(70)	(72)
Amortization of unrecognized loss	100	129
Net periodic benefit cost	$ 1,006	$ 1,270