Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,801	$ 3,440	$ 11,452	$ 9,822
Interest cost	25,030	24,807	75,299	72,948
Expected return on plan assets	(32,098)	(30,918)	(96,260)	(93,044)
Amortization of prior service credit	(1,749)	(1,720)	(5,241)	(5,223)
Amortization of actuarial loss	17,314	14,417	52,809	39,766
Net periodic benefit cost	12,298	10,026	38,059	24,269
Other Post-retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost
Interest cost	66	123	200	356
Expected return on plan assets
Amortization of prior service credit	(232)	(168)	(698)	(698)
Amortization of actuarial loss	307	415	941	1,282
Net periodic benefit cost	$ 141	$ 370	$ 443	$ 940