Components of Net Periodic Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,724	$ 3,770
Interest cost	6,909	7,170
Expected return on plan assets	(10,512)	(10,214)
Amortization of prior service cost (credit)	476	511
Amortization of actuarial losses	3,088	1,992
Net periodic benefit cost	3,685	3,229
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	227	259
Interest cost	569	657
Amortization of prior service cost (credit)	(654)	(642)
Amortization of actuarial losses	89	142
Net periodic benefit cost	$ 231	$ 416