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Effects of One-percentage Point Change in Assumed Health Care Cost Trend Rates (Detail) (Other Postretirement Benefit Plans, Defined Benefit, USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Other Postretirement Benefit Plans, Defined Benefit
 
Defined Benefit Plan Disclosure [Line Items]  
Postretirement benefit obligation, 1.0% Increase $ 6.8
Total postretirement service and interest cost components, 1.0% Increase 0.5
Postretirement benefit obligation, 1.0% Decrease (5.7)
Total postretirement service and interest cost components, 1.0% Decrease $ (0.5)