Amounts Included in AOCI that are Expected to be Amortized into Net Periodic Benefit Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ (16,872)
Prior service (cost) credit	(1,258)
Transitional asset (liability)	535
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Total	(17,595)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	(1,846)
Prior service (cost) credit	173
Transitional asset (liability)	(8)
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Total	$ (1,681)