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Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value $ 19,943 $ 26,601
Current assets - Fair market value of derivatives [Member] | Not Designated as Hedging Instrument [Member] | Crude Oil [Member]    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 19,943 26,588
Current assets - Fair market value of derivatives [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas [Member]    
Derivatives, Fair Value [Line Items]    
Asset Fair Value   1,262
Current liabilities - Fair value of derivatives [Member] | Not Designated as Hedging Instrument [Member] | Crude Oil [Member]    
Derivatives, Fair Value [Line Items]    
Liability Fair Value   (1,242)
Current liabilities - Fair value of derivatives [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas [Member]    
Derivatives, Fair Value [Line Items]    
Liability Fair Value   (7)
Balance Sheet Current [Member] | Not Designated as Hedging Instrument [Member]    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 19,943 27,850
Liability Fair Value   (1,249)
Net Derivative Fair Value 19,943 26,601
Balance Sheet Current [Member] | Not Designated as Hedging Instrument [Member] | Crude Oil [Member]    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value $ 19,943 25,346
Balance Sheet Current [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas [Member]    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value   $ 1,255