Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on fair value adjustments	$ (8,648)	$ (7,914)
Total gain (loss) on derivative contracts	(932)	2,429
Crude Oil [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain on settlements	7,507	9,952
Net gain (loss) on fair value adjustments	(9,137)	(7,789)
Total gain (loss) on derivative contracts	(1,630)	2,163
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain on settlements	209	391
Net gain (loss) on fair value adjustments	489	(125)
Total gain (loss) on derivative contracts	$ 698	$ 266