Components of Net Periodic Other Postretirement Benefits Costs (Detail) (Other Postretirement Benefits, USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 228	$ 259	$ 455	$ 518
Interest cost	571	658	1,140	1,315
Amortization of prior service credit	(654)	(643)	(1,308)	(1,285)
Amortization of actuarial losses	53	142	142	284
Net periodic benefit cost	$ 198	$ 416	$ 429	$ 832