Components of Net Periodic Benefit Expense (Detail) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,915	$ 5,282	$ 17,763	$ 15,738
Interest cost	7,919	7,480	23,855	22,361
Expected return on plan assets	(6,840)	(5,933)	(20,634)	(17,675)
Amortization of prior service cost	337	387	1,020	1,158
Amortization of transitional asset	(51)	(127)	(155)	(383)
Recognized actuarial loss	2,543	2,995	7,661	8,948
Defined Benefit Plan, Gross Periodic Benefit Cost, Total	9,823	10,084	29,510	30,147
Termination benefits expense	700	0	700	0
Curtailment expense (gain)	1,105	0	1,105	0
Net periodic benefit expense	11,628	10,084	31,315	30,147
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,289	921	3,803	2,729
Interest cost	1,719	1,474	5,084	4,379
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(66)	(67)	(196)	(197)
Amortization of transitional asset	3	0	7	0
Recognized actuarial loss	786	596	2,326	1,770
Defined Benefit Plan, Gross Periodic Benefit Cost, Total	3,731	2,924	11,024	8,681
Termination benefits expense	0	0	0	0
Curtailment expense (gain)	(605)	0	(605)	0
Net periodic benefit expense	$ 3,126	$ 2,924	$ 10,419	$ 8,681