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Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Oil Floors [Member]
   
Fair value of plan assets    
Fair value of plan assets, Derivatives $ 0.8 $ 0.1 [1],[2]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Oil Floors [Member]
   
Fair value of plan assets    
Fair value of plan assets, Derivatives      
Significant Other Observable Inputs (Level 2) [Member] | Oil Floors [Member]
   
Fair value of plan assets    
Fair value of plan assets, Derivatives 0.8 0.1
Significant Unobservable Inputs (Level 3) [Member] | Oil Floors [Member]
   
Fair value of plan assets    
Fair value of plan assets, Derivatives      
Senior notes due 2017 [Member]
   
Fair value of senior notes    
Fair value of senior notes 257.5 254.8
Carrying value of senior notes 250.0 250.9
Senior notes due 2020 [Member]
   
Fair value of senior notes    
Fair value of senior notes 243.6 239.6
Carrying value of senior notes 221.9 221.9
Senior notes due 2022 [Member]
   
Fair value of senior notes    
Fair value of senior notes 255.0 252.8
Carrying value of senior notes $ 247.9 $ 247.9
[1] (2) Unaudited
[2] (1) $.01 par value.