Annual report pursuant to Section 13 and 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
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on fair value adjustments $ (10,429) $ (38,135) $ (6,658)
Total gain (loss) on derivative contracts (18,901) (20,233) 28,358
Not Designated as Hedging Instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Total gain (loss) on derivative contracts (18,901) (20,233) 28,358
Not Designated as Hedging Instrument | Oil      
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on settlements (9,067) 17,801 33,299
Net gain (loss) on fair value adjustments (11,426) (37,543) (5,403)
Total gain (loss) on derivative contracts (20,493) (19,742) 27,896
Not Designated as Hedging Instrument | Natural gas      
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on settlements 594 102 1,717
Net gain (loss) on fair value adjustments 998 (593) (1,255)
Total gain (loss) on derivative contracts $ 1,592 $ (491) $ 462