Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Net gain on fair value adjustments	$ 3,978	$ 17,794
Total gain (loss) on oil derivatives	(4,481)	15,303
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) on oil derivatives	(4,481)	15,303
Not Designated as Hedging Instrument | Oil
Derivative Instruments, Gain (Loss) [Line Items]
Net loss on settlements	(8,916)	(2,524)
Net gain on fair value adjustments	4,067	17,266
Total gain (loss) on oil derivatives	(4,849)	14,742
Not Designated as Hedging Instrument | Natural gas
Derivative Instruments, Gain (Loss) [Line Items]
Net loss on settlements	457	33
Net gain on fair value adjustments	(89)	528
Total gain (loss) on oil derivatives	$ 368	$ 561