Net Periodic Pension and Postretirement Benefit Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1,086	$ 1,208	$ 3,260	$ 3,625
Expected return on plan assets	(1,173)	(1,333)	(3,519)	(4,000)
Amortization of net actuarial loss	1,170	619	3,510	1,857
Total net periodic pension expense	1,083	494	3,251	1,482
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	14	43	43
Interest cost	254	257	761	772
Amortization of net actuarial loss	176		528
Amortization of prior service credit	(109)		(326)
Amortization of curtailment gain	(21)	(61)	(65)	(183)
Total net postretirement benefit expense	$ 314	$ 210	$ 941	$ 632