Derivatives and Risk Management (Cash Flow Hedges on Volumes of Natural Gas Production) (Details)	Mar. 31, 2012
Year 2012 [Member] | Costless Collar [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	60.5
Year 2012 [Member] | Fixed Price Swaps [Member]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	139.2
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