Derivatives and Risk Management (Narratives) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Derivative [Line Items]
Accumulated other comprehensive income, net gain related to hedging activities	$ 72.1
Deferred income tax liability, accumulated other comprehensive income	48.1
Cash flow hedge after-tax net gain to be transferred from accumulated other comprehensive income to earnings during the next twelve months	68.6
Period of time needed to transfers net gain from accumulated other comprehensive income to earnings, months	12 months
Year 2013 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment	6,500,000,000
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), Basis Swaps Not Qualifying for Hedge Accounting Treatment	16,700,000,000
Volume of Natural Gas Production (in Bcf), Fixed Price Swaps	232,700,000,000
Year 2015 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment	900,000,000
Volume of Natural Gas Production (in Bcf), Call Options Not Qualifying for Hedge Accounting Treatment	199,800,000,000
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
Interest Rate Swaps [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount	$ 170