Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on fair value adjustments			$ (29,696)	$ 15,608
Total gain (loss) on oil & natural gas derivatives	$ (34,339)	$ (14,162)	(55,374)	11,636
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) on oil & natural gas derivatives	(34,339)	(14,162)	(55,374)	11,636
Not Designated as Hedging Instrument | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	(9,306)	(1,373)	(26,353)	(4,213)
Net gain (loss) on fair value adjustments	(24,476)	(12,811)	(28,720)	14,584
Total gain (loss) on oil & natural gas derivatives	(33,782)	(14,184)	(55,073)	10,371
Not Designated as Hedging Instrument | Natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	67	159	675	241
Net gain (loss) on fair value adjustments	(624)	(137)	(976)	1,024
Total gain (loss) on oil & natural gas derivatives	$ (557)	$ 22	$ (301)	$ 1,265