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
Mar. 31, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Asset Fair Value $ 4,210 $ 406
Liability Fair Value (28,854) (29,028)
Net Derivative Fair Value (24,644) (28,622)
Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 4,210 406
Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (25,912) (27,744)
Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (2,942) (1,284)
Natural gas | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 317 406
Natural gas | Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value 0 0
Natural gas | Current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value 317 406
Oil | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 3,893 0
Oil | Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (25,912) (27,744)
Oil | Current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value (22,019) (27,744)
Oil | Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 0 0
Oil | Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (2,942) (1,284)
Oil | Non-current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value $ (2,942) $ (1,284)