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Benefit Costs Included in Accumulated Other Comprehensive Loss that have not yet Recognized in Net Periodic Pension Cost (Detail)

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Benefit Costs Included in Accumulated Other Comprehensive Loss that have not yet Recognized in Net Periodic Pension Cost (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]      
Accumulated other comprehensive loss $ 276.4 $ 300.3 $ 241.3
Pension Benefits [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Prior service cost, net of accumulated taxes of $3.5 and $0.9 in 2012 and 2011, respectively, for pension benefits and $(0.3) and $(0.4) in 2012 and 2011, respectively, for other benefits 6.0 1.8  
Net actuarial loss, net of accumulated taxes of $151.5 and $165.6 in 2012 and 2011, respectively, for pension benefits and $4.6 and $6.0 in 2012 and 2011, respectively, for other benefits 262.9 288.6  
Accumulated other comprehensive loss 268.9 290.4  
Other Benefits [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Prior service cost, net of accumulated taxes of $3.5 and $0.9 in 2012 and 2011, respectively, for pension benefits and $(0.3) and $(0.4) in 2012 and 2011, respectively, for other benefits (0.4) (0.6)  
Net actuarial loss, net of accumulated taxes of $151.5 and $165.6 in 2012 and 2011, respectively, for pension benefits and $4.6 and $6.0 in 2012 and 2011, respectively, for other benefits 7.9 10.5  
Accumulated other comprehensive loss $ 7.5 $ 9.9