Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on fair value adjustments	$ (66,970)	$ 3,978
Total gain (loss) on oil & natural gas derivatives	(67,260)	(4,481)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) on oil & natural gas derivatives	(67,260)	(4,481)
Not Designated as Hedging Instrument | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	(1,542)	(8,916)
Net gain (loss) on fair value adjustments	(66,827)	4,067
Total gain (loss) on oil & natural gas derivatives	(68,369)	(4,849)
Not Designated as Hedging Instrument | Natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	1,252	457
Net gain (loss) on fair value adjustments	(143)	(89)
Total gain (loss) on oil & natural gas derivatives	$ 1,109	$ 368