Derivatives and Risk Management (Narratives) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Accumulated other comprehensive income, net gain related to hedging activities	$ 172.2
Deferred income tax liability, accumulated other comprehensive income	112.9
Cash flow hedge after-tax net gain to be transferred from accumulated other comprehensive income to earnings during the next twelve months	167.2
Realized Gain From Settled Contracts	$ 631.5	$ 320.1
Year 2013 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment	30.1
Year 2014 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Basis Swaps Not Qualifying for Hedge Accounting Treatment	9.1
Year 2015 [Member]
Derivative [Line Items]
Volume of Natural Gas Production (in Bcf), Call Options Not Qualifying for Hedge Accounting Treatment	18.3