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Schedule of Assumed Health Care Cost Trend Rates For Its Postretirement Benefits (Detail)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Health Care Cost Trend Rates Assumptions [Line Items]      
Health care cost trend rate assumed for next year 8.00% 8.00% 7.50%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 5.00% 5.50% 5.00%
Year that the rate reaches the ultimate trend rate 2020 2016 2016