Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on fair value adjustments			$ (4,594)	$ 28,555
Total gain (loss) on oil & natural gas derivatives	$ (16,554)	$ 10,494	(21,036)	25,797
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) on oil & natural gas derivatives	(16,554)	10,494	(21,036)	25,797
Not Designated as Hedging Instrument | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Net loss on settlements	(8,131)	(315)	(17,049)	(2,840)
Net gain (loss) on fair value adjustments	(8,311)	10,128	(4,243)	27,394
Total gain (loss) on oil & natural gas derivatives	(16,442)	9,813	(21,292)	24,554
Not Designated as Hedging Instrument | Natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Net loss on settlements	151	48	607	82
Net gain (loss) on fair value adjustments	(263)	633	(351)	1,161
Total gain (loss) on oil & natural gas derivatives	$ (112)	$ 681	$ 256	$ 1,243