Components of Net Periodic Benefit Expense (Detail) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,952	$ 5,197	$ 11,848	$ 10,456
Interest cost	7,943	7,433	15,936	14,881
Expected return on plan assets	(6,869)	(5,891)	(13,794)	(11,742)
Amortization of prior service cost	339	384	683	771
Amortization of transitional asset	(53)	(129)	(104)	(256)
Recognized actuarial loss	2,542	2,988	5,118	5,953
Net periodic benefit expense	9,854	9,982	19,687	20,063
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,290	920	2,514	1,808
Interest cost	1,718	1,474	3,365	2,905
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(66)	(66)	(130)	(130)
Amortization of transitional asset	2	0	4	0
Recognized actuarial loss	787	596	1,540	1,174
Net periodic benefit expense	$ 3,731	$ 2,924	$ 7,293	$ 5,757