Annual report pursuant to Section 13 and 15(d)

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

v3.6.0.2
Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on fair value adjustments $ (38,135) $ (6,658) $ 27,650
Total gain (loss) on derivative contracts (20,233) 28,358 31,736
Not Designated as Hedging Instrument [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Total gain (loss) on derivative contracts (20,233) 28,358 31,736
Crude Oil [Member] | Not Designated as Hedging Instrument [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on settlements 17,801 33,299 4,170
Net gain (loss) on fair value adjustments (37,543) (5,403) 26,383
Total gain (loss) on derivative contracts (19,742) 27,896 30,553
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on settlements 102 1,717 (84)
Net gain (loss) on fair value adjustments (593) (1,255) 1,267
Total gain (loss) on derivative contracts $ (491) $ 462 $ 1,183