Benefits Expected to Be Paid from Pension and Other Postretirement Benefit Plans, Which Reflect Future Years of Services and Medicare Subsidy Expected to Be Received (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
2012	$ 27,363
2013	28,769
2014	30,096
2015	31,480
2016	32,616
2017 - 2021	180,587
Defined Benefit Plan, Expected Future Benefit Payments, Total	330,911
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
2012	4,311
2013	4,380
2014	4,157
2015	3,837
2016	3,772
2017 - 2021	17,529
Defined Benefit Plan, Expected Future Benefit Payments, Total	$ 37,986