Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments) (Details)

v3.8.0.1
Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Asset Fair Value $ 4,454 $ 103
Liability Fair Value (7,039) (18,296)
Net Derivative Fair Value (2,585) (18,193)
Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 3,333 103
Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (6,380) (18,268)
Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 1,121  
Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (659) (28)
Natural gas | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 431 0
Natural gas | Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value 0 (593)
Natural gas | Current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value 431 (593)
Oil | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 2,902 103
Oil | Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (6,380) (17,675)
Oil | Current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value (3,478) (17,572)
Oil | Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 1,121 0
Oil | Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (659) (28)
Oil | Non-current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value $ 462 $ (28)