Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 13,857	$ 13,428	$ 13,634
Interest cost	28,519	28,200	27,657
Expected return on plan assets	(40,817)	(35,524)	(25,254)
Amortization of prior service cost (credit)	2,042	2,065	2,221
Amortization of actuarial losses	8,171	8,400	9,410
Net curtailment gain	(449)
Net periodic benefit cost	11,323	16,569	27,668
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	862	918	781
Interest cost	2,441	2,782	2,916
Amortization of prior service cost (credit)	(2,582)	(2,567)	(2,564)
Amortization of actuarial losses	134	287	182
Net periodic benefit cost	$ 855	$ 1,420	$ 1,315