Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details)

v3.8.0.1
Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]    
Net gain on fair value adjustments $ 3,978 $ 17,794
Total gain (loss) on oil derivatives (4,481) 15,303
Not Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Total gain (loss) on oil derivatives (4,481) 15,303
Not Designated as Hedging Instrument | Oil    
Derivative Instruments, Gain (Loss) [Line Items]    
Net loss on settlements (8,916) (2,524)
Net gain on fair value adjustments 4,067 17,266
Total gain (loss) on oil derivatives (4,849) 14,742
Not Designated as Hedging Instrument | Natural gas    
Derivative Instruments, Gain (Loss) [Line Items]    
Net loss on settlements 457 33
Net gain on fair value adjustments (89) 528
Total gain (loss) on oil derivatives $ 368 $ 561