Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 14,788	$ 13,857	$ 13,428
Interest cost	27,363	28,519	28,200
Expected return on plan assets	(46,967)	(40,817)	(35,524)
Amortization of prior service cost (credit)	1,797	2,042	2,065
Amortization of actuarial losses	12,168	8,171	8,400
Net curtailment gain	(706)	(449)
Net periodic benefit cost	8,443	11,323	16,569
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	830	862	918
Interest cost	2,051	2,441	2,782
Amortization of prior service cost (credit)	(2,616)	(2,582)	(2,567)
Amortization of actuarial losses	12	134	287
Net periodic benefit cost	$ 277	$ 855	$ 1,420