Annual report pursuant to Section 13 and 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
Dec. 31, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Asset Fair Value $ 406 $ 103
Liability Fair Value (29,028) (18,296)
Total net liabilities (28,622) (18,193)
Current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 406 103
Current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (27,744) (18,268)
Non-current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (1,284) (28)
Natural gas | Current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 406 0
Natural gas | Current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value 0 (593)
Natural gas | Derivative Current    
Derivatives, Fair Value [Line Items]    
Total net liabilities 406 (593)
Oil | Current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 0 103
Oil | Current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (27,744) (17,675)
Oil | Derivative Current    
Derivatives, Fair Value [Line Items]    
Total net liabilities (27,744) (17,572)
Oil | Non-current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 0 0
Oil | Non-current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (1,284) (28)
Oil | Derivative Noncurrent    
Derivatives, Fair Value [Line Items]    
Total net liabilities $ (1,284) $ (28)