Derivatives and Risk Management (Cash Flow Hedges on Volumes of Natural Gas Production) (Details)	Sep. 30, 2013 ft3
Year 2013 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Fixed Price Swaps	84,400,000,000
Year 2014 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Fixed Price Swaps	232,700,000,000
Not Designated as Hedging Instrument [Member] | Year 2014 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Fixed Price Swaps	181,600,000,000
Designated as Hedging Instrument [Member] | Year 2013 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Fixed Price Swaps	84,400,000,000
Designated as Hedging Instrument [Member] | Year 2014 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Fixed Price Swaps	51,100,000,000