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Fair Value (Details) (USD $)
Sep. 30, 2014
Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying value of senior notes $ 550,000,000 $ 550,000,000
Senior Notes Due 2020 [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of senior notes 284,000,000 316,700,000
Carrying value of senior notes 300,000,000 300,000,000
Amount of difference between fair value and carrying value (16,000,000) 16,700,000
Senior Notes Due 2021 [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of senior notes 206,100,000 234,700,000
Carrying value of senior notes 250,000,000 250,000,000
Amount of difference between fair value and carrying value $ (43,900,000) $ (15,300,000)