Derivatives and Risk Management (Cash Flow Hedges on Volumes of Natural Gas Production) (Details)	Dec. 31, 2011
Year 2012 [Member] | Costless Collar [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	80.50
Year 2012 [Member] | Fixed Price Swaps [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	185.90
Year 2013 [Member] | Costless Collar [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	0.00
Year 2013 [Member] | Fixed Price Swaps [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	185.20