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Derivative Instruments (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2012
Dec. 31, 2011
Derivative Instruments      
Maximum Remaining Maturity of Foreign Currency Derivatives 6 6  
Senior notes, 4.5 percent due 2021
     
Derivative Instruments      
Debt instrument, face amount $ 400 $ 400  
Forward exchange contracts
     
Derivative Instruments      
Notional amounts of derivatives 7.7 7.7 9.8
Derivatives designated as hedging instruments | Interest-rate swap
     
Derivative Instruments      
Notional amounts of derivatives     400.0
Number of swap agreements     4
Net settlement benefit due to reset of variable rates semi-annually at each net settlement date 1.8 5.1  
(Level 2) | Measured on a recurring basis | Interest-rate swap
     
Derivative Instruments      
Derivative net asset (liability) position $ 20.6 $ 20.6 $ 3.3