Quarterly report pursuant to Section 13 or 15(d)

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

v3.19.1
Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Derivatives, Fair Value [Line Items]    
Asset Fair Value $ 11,757 $ 65,114
Liability Fair Value (31,533) (17,920)
Net Derivative Fair Value (19,776) 47,194
Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 11,372 65,114
Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (24,550) (10,480)
Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 385  
Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (6,983) (7,440)
Oil | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 7,473 60,097
Oil | Current liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (24,550) (10,480)
Oil | Current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value (17,077) 49,617
Oil | Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 385 0
Oil | Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (6,190) (5,672)
Oil | Non-current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value (5,805) (5,672)
Natural gas | Current assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 3,899 5,017
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 3,899 5,017
Natural gas | Long-term assets: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 0 0
Natural gas | Long-term liabilities: Fair value of derivatives    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (793) (1,768)
Natural gas | Non-current    
Derivatives, Fair Value [Line Items]    
Net Derivative Fair Value $ (793) $ (1,768)