Derivatives and Risk Management (Cash Flow Hedges on Volumes of Natural Gas Production) (Details)	Sep. 30, 2011
Year 2011 [Member] | Costless Collar [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	15.6
Year 2011 [Member] | Fixed Price Swaps [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	64.7
Year 2012 [Member] | Costless Collar [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	80.5
Year 2012 [Member] | Fixed Price Swaps [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	185.9
Year 2013 [Member] | Costless Collar [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	0
Year 2013 [Member] | Fixed Price Swaps [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	185.2