Components of Net Periodic Cost for Defined Benefit Pension Plans and Other Postretirement Benefit Plan (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	$ 1,176	$ 934	$ 2,321	$ 1,867
Interest cost	3,048	2,897	6,105	5,796
Expected return on plan assets	(2,649)	(2,356)	(5,324)	(4,711)
Recognized net actuarial loss	2,148	1,603	4,223	3,205
Amortization of prior service cost (credit)	48	62	96	124
Settlement	(28)
Net periodic cost	3,743	3,140	7,421	6,281
Other Postretirement Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost	126	90	265	179
Interest cost	345	283	680	566
Recognized net actuarial loss	45		74
Amortization of prior service cost (credit)	(25)	(25)	(50)	(50)
Net periodic cost	$ 491	$ 348	$ 969	$ 695