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Pension and Postretirement Benefit Plans - Healthcare Cost Trend Rate Assumptions (Detail) (Postretirement Plans [Member])
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Postretirement Plans [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Healthcare cost trend rate assumed for next year 7.80% 8.00%
Rate to which the cost trend rate is assumed to decline 4.50% 4.50%
Year that the rate reaches the ultimate trend rate 2027 2027