Weighted-Average Actuarial Assumptions (Parenthetical) (Detail) (Other Postretirement Benefit Plans, Defined Benefit)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Other Postretirement Benefit Plans, Defined Benefit
Schedule of Net Periodic Benefit Costs and Assumptions for Defined Benefit Postretirement Plans [Line Items]
Health care cost, ultimate trend rates	4.20%	3.90%	4.50%
Health care cost trend rates, year when the ultimate trend rate is expected to be reached	75 years