Amounts Included in Accumulated Other Comprehensive Income Not Recognized in Net Periodic Benefit Expense (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ (238,684)
Prior service (cost) credit	(3,987)
Transitional asset (liability)	1,062
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax, Total	(241,609)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	(38,706)
Prior service (cost) credit	957
Transitional asset (liability)	(8)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax, Total	$ (37,757)