Derivatives and Risk Management (Narratives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Derivative [Line Items]
Accumulated other comprehensive income, net gain related to hedging activities	$ 78.7
Deferred income tax liability, accumulated other comprehensive income	52.5
Cash flow hedge after-tax net gain to be transferred from accumulated other comprehensive income to earnings during the next twelve months	$ 76.3
Period of time needed to transfers net gain from accumulated other comprehensive income to earnings, months	12 months
Fair value hedges	For fair value hedges, the gain or loss on the derivative instrument as well as the offsetting gain or loss on the hedged item are recognized in earnings immediately. As of March 31, 2013 and December 31, 2012, the Company had no material fair value hedges.
Year 2013 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	139,600,000,000
Year 2014 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment	22,300,000,000
Volume of Natural Gas Production (in Bcf), Cash Flow Hedges	18,300,000,000
Year 2015 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment	10,900,000,000
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	181,500,000,000