Annual report pursuant to Section 13 and 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
Dec. 31, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Asset Fair Value $ 65,114 $ 406
Liability Fair Value (17,920) (29,028)
Total net assets (liabilities) 47,194 (28,622)
Not Designated as Hedging Instrument    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 65,114 406
Liability Fair Value (17,920) (29,028)
Total net assets (liabilities) 47,194 (28,622)
Not Designated as Hedging Instrument | Current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 65,114 406
Not Designated as Hedging Instrument | Current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (10,480) (27,744)
Not Designated as Hedging Instrument | Non-current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (7,440) (1,284)
Not Designated as Hedging Instrument | Oil | Current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 60,097 0
Not Designated as Hedging Instrument | Oil | Current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (10,480) (27,744)
Not Designated as Hedging Instrument | Oil | Derivative Current    
Derivatives, Fair Value [Line Items]    
Total net assets (liabilities) 49,617 (27,744)
Not Designated as Hedging Instrument | Oil | Non-current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 0 0
Not Designated as Hedging Instrument | Oil | Non-current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (5,672) (1,284)
Not Designated as Hedging Instrument | Oil | Derivative Noncurrent    
Derivatives, Fair Value [Line Items]    
Total net assets (liabilities) (5,672) (1,284)
Not Designated as Hedging Instrument | Natural gas | Current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 5,017 406
Not Designated as Hedging Instrument | Natural gas | Current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value 0 0
Not Designated as Hedging Instrument | Natural gas | Derivative Current    
Derivatives, Fair Value [Line Items]    
Total net assets (liabilities) 5,017 406
Not Designated as Hedging Instrument | Natural gas | Non-current Assets    
Derivatives, Fair Value [Line Items]    
Asset Fair Value 0 0
Not Designated as Hedging Instrument | Natural gas | Non-current Liabilities    
Derivatives, Fair Value [Line Items]    
Liability Fair Value (1,768) 0
Not Designated as Hedging Instrument | Natural gas | Derivative Noncurrent    
Derivatives, Fair Value [Line Items]    
Total net assets (liabilities) $ (1,768) $ 0