Components of Net Periodic Benefit Expense (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
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,030	$ 5,915	$ 17,953	$ 17,763
Interest cost	7,549	7,919	22,386	23,855
Expected return on plan assets	(6,520)	(6,840)	(19,345)	(20,634)
Amortization of prior service cost	313	337	938	1,020
Amortization of transitional asset	112	(51)	339	(155)
Recognized actuarial loss	3,846	2,543	11,460	7,661
Defined Benefit Plan, Gross Periodic Benefit Cost, Total	11,330	9,823	33,731	29,510
Special termination benefits	0	700	6,170	700
Curtailment expense (gain)	0	1,105	0	1,105
Net periodic benefit expense	11,330	11,628	39,901	31,315
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,049	1,289	3,139	3,803
Interest cost	1,342	1,719	4,133	5,084
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(42)	(66)	(131)	(196)
Amortization of transitional asset	2	3	6	7
Recognized actuarial loss	453	786	1,394	2,326
Defined Benefit Plan, Gross Periodic Benefit Cost, Total	2,804	3,731	8,541	11,024
Special termination benefits	0	0	0	0
Curtailment expense (gain)	0	(605)	0	(605)
Net periodic benefit expense	$ 2,804	$ 3,126	$ 8,541	$ 10,419