Derivatives and Risk Management (Cash Flow Hedges on Volumes of Natural Gas Production) (Details)	Sep. 30, 2012
Year 2012 [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	46.7
Year 2012 [Member] | Costless Collar [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	20.2
Year 2013 [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	185.6
Year 2013 [Member] | Costless Collar [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges