Derivative Instruments and Hedging Activities (Gain or loss on derivative contracts) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on mark-to-market adjustments	$ (1,639)	$ (1,039)
Total loss on derivative instruments	(2,513)	(418)
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	(102)	49
Net gain (loss) on mark-to-market adjustments	(190)	(388)
Total loss on derivative instruments	(292)	(339)
Crude Oil [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	(773)	573
Net gain (loss) on mark-to-market adjustments	(1,448)	(652)
Total loss on derivative instruments	$ (2,221)	$ (79)