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 Benefit [Member]
Components of Net Periodic Benefit Cost
Service cost	$ 5,587	$ 5,190	$ 16,762	$ 15,571
Interest cost	10,798	10,595	32,395	31,787
Expected return on plan assets	(12,195)	(12,370)	(36,585)	(37,110)
Amortization of prior service cost	69	85	206	255
Amortization of actuarial loss	4,882	2,918	14,645	8,753
Net periodic benefit cost	9,141	6,418	27,423	19,256
Pretax reclassification from AOCI included in net periodic benefit cost	4,951	3,003	14,851	9,008
Other Postretirement Benefits [Member]
Components of Net Periodic Benefit Cost
Service cost	1,166	1,197	3,499	3,592
Interest cost	1,563	1,613	4,687	4,838
Amortization of prior service cost	(169)	(169)	(506)	(506)
Amortization of actuarial loss	287	288	862	862
Net periodic benefit cost	2,847	2,929	8,542	8,786
Pretax reclassification from AOCI included in net periodic benefit cost	$ 118	$ 119	$ 356	$ 356