Annual report pursuant to Section 13 and 15(d)

Effect of One-Percentage Point Change in Assumed Healthcare Cost Trend Rates (Detail)

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Effect of One-Percentage Point Change in Assumed Healthcare Cost Trend Rates (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]  
Effect of 1-percentage point increase on total service and interest cost components $ 0.2
Effect of 1-percentage point increase on accumulated postretirement benefit obligation 3.0
Effect of 1-percentage point decrease on total service and interest cost components (0.2)
Effect of 1-percentage point decrease on accumulated postretirement benefit obligation $ (2.6)