Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,976	$ 3,833
Interest cost	22,407	25,144
Expected return on plan assets	(33,518)	(32,094)
Amortization of prior service credit	(1,894)	(1,748)
Amortization of actuarial loss	21,112	17,755
Net periodic benefit cost	13,083	12,890
Other Post-retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost
Interest cost	38	67
Expected return on plan assets
Amortization of prior service credit	(239)	(233)
Amortization of actuarial loss	281	317
Net periodic benefit cost	$ 80	$ 151