Derivatives and Risk Management (Narratives) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Derivative [Line Items]
Accumulated other comprehensive income, net gain related to hedging activities	$ 119.7
Deferred income tax liability, accumulated other comprehensive income	79.8
Cash flow hedge after-tax net gain to be transferred from accumulated other comprehensive income to earnings during the next twelve months	112.5
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	14,100,000,000
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	168,800,000,000
Year 2014 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment	15,200,000,000
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	51,100,000,000
Year 2015 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Call Options Not Qualifying for Hedge Accounting Treatment	199,800,000,000
Fixed Price Swaps [Member] | Not Designated as Hedging Instrument [Member] | Year 2014 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	181,600,000,000
Interest Rate Swaps [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount	$ 170