Quarterly report pursuant to Section 13 or 15(d)

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

v3.10.0.1
Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments) (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Asset Fair Value $ 2,299 $ 0
Not Designated as Hedging Instrument    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 13,868 406
Liability Fair Value (47,084) (29,028)
Net Derivative Fair Value (33,216) (28,622)
Not Designated as Hedging Instrument | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 11,569 406
Not Designated as Hedging Instrument | Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (35,948) (27,744)
Not Designated as Hedging Instrument | Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 2,299  
Not Designated as Hedging Instrument | Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (11,136) (1,284)
Not Designated as Hedging Instrument | Natural gas | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 391 406
Not Designated as Hedging Instrument | Natural gas | Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (35) 0
Not Designated as Hedging Instrument | Natural gas | Current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value 356 406
Not Designated as Hedging Instrument | Natural gas | Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 0 0
Not Designated as Hedging Instrument | Natural gas | Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (302) 0
Not Designated as Hedging Instrument | Natural gas | Non-current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value (302) 0
Not Designated as Hedging Instrument | Oil | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 11,178 0
Not Designated as Hedging Instrument | Oil | Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (35,913) (27,744)
Not Designated as Hedging Instrument | Oil | Current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value (24,735) (27,744)
Not Designated as Hedging Instrument | Oil | Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 2,299 0
Not Designated as Hedging Instrument | Oil | Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (10,834) (1,284)
Not Designated as Hedging Instrument | Oil | Non-current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value $ (8,535) $ (1,284)