Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities (Gain or loss on derivative contracts) (Details)

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Derivative Instruments and Hedging Activities (Gain or loss on derivative contracts) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]        
Net gain (loss) on fair value adjustments     $ (7,635) $ 5,728
Total gain (loss) on derivative instruments $ 23,283 $ 9,944 17,463 2,746
Crude Oil [Member] | Not Designated as Hedging Instrument [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Net gain (loss) on settlements 9,399 (497) 23,863 (2,838)
Net gain (loss) on fair value adjustments 13,758 10,351 (6,787) 5,805
Total gain (loss) on derivative instruments 23,157 9,854 17,076 2,967
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Net gain (loss) on settlements 390 35 1,235 (144)
Net gain (loss) on fair value adjustments (264) 55 (848) (77)
Total gain (loss) on derivative instruments $ 126 $ 90 $ 387 $ (221)