Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on fair value adjustments	$ (6,658)	$ 27,650	$ (2,730)
Total gain (loss) on derivative contracts	28,358	31,736	(1,360)
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) on derivative contracts	28,358	31,736	(1,360)
Crude Oil [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	33,299	4,170	1,518
Net gain (loss) on fair value adjustments	(5,403)	26,383	(2,960)
Total gain (loss) on derivative contracts	27,896	30,553	(1,442)
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	1,717	(84)	(148)
Net gain (loss) on fair value adjustments	(1,255)	1,267	230
Total gain (loss) on derivative contracts	$ 462	$ 1,183	$ 82