Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on fair value adjustments	$ 17,794	$ (8,648)
Total gain (loss) on derivative contracts	15,303	(932)
Crude Oil [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	(2,524)	7,507
Net gain (loss) on fair value adjustments	17,266	(9,137)
Total gain (loss) on derivative contracts	14,742	(1,630)
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	33	209
Net gain (loss) on fair value adjustments	528	489
Total gain (loss) on derivative contracts	$ 561	$ 698