Pension Plans, Other Postretirement Benefits, and Defined Contribution Plans - Weighted-Average Assumptions for Pension and OPEB Table (Detail)	Dec. 31, 2011	Dec. 31, 2010
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.50%	4.75%
Rates of increase in compensation levels	4.50%	5.00%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.75%	5.25%
Rates of increase in compensation levels	4.50%	5.00%