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Fair Value (Details) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Carrying value of senior notes $ 550,000 $ 550,000
Amount of difference between fair value and carrying value (37,125) (51,375)
Senior Notes Due 2020 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Carrying value of senior notes 300,000 300,000
Amount of difference between fair value and carrying value (5,250) (15,750)
Senior Notes Due 2021 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Carrying value of senior notes 250,000 250,000
Amount of difference between fair value and carrying value (31,875) (35,625)
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of senior notes 512,875 498,625
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member] | Senior Notes Due 2020 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of senior notes 294,750 284,250
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member] | Senior Notes Due 2021 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of senior notes $ 218,125 $ 214,375