Derivative Instruments and Hedging Activities (Gain or loss on derivative contracts) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on fair value adjustments	$ (7,914)	$ (1,639)
Total gain (loss) on derivative instruments	2,429	(2,513)
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	391	(102)
Net gain (loss) on fair value adjustments	(125)	(190)
Total gain (loss) on derivative instruments	266	(292)
Crude Oil [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	9,952	(773)
Net gain (loss) on fair value adjustments	(7,789)	(1,448)
Total gain (loss) on derivative instruments	$ 2,163	$ (2,221)