Other Events (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended				1 Months Ended		3 Months Ended	12 Months Ended	3 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2009 International Commodities Operation	Jun. 30, 2009 Uranium market participant	Apr. 30, 2009 Houston-Based Gas and Other Trading Operations	Jun. 30, 2009 Houston-Based Gas and Other Trading Operations	Dec. 31, 2009 Houston-Based Gas and Other Trading Operations	Dec. 31, 2009 Other Nonregulated Divestiture
Equity in earnings of investments accounted for under the equity method of accounting
Net Gain (Loss) on Divestitures	$ 57.3	$ 245.8	$ (468.8)	$ (334.5)	$ (27.2)			$ (102.5)	$ (4.6)
Reclassification of (losses) on previously designated cash-flow hedge contracts	(180.8)	(582.4)	(1,499.4)	(165.7)
Workforce reduction costs			12.6	10.9
Other costs			31.2	17.6				7.0
Payment for contracts assigned				90
Net cash (payment) receipt to buyer upon completion of TRS				(2.5)			91.9	91.9
Net liability resulting from cash payment to buyer and acquiring matching nonderivative contracts				75
Contract and portfolio acquisitions	(3.7)	(208.3)	(2,153.7)	(866.3)				(1,287.4)
Proceeds from contract and portfolio acquisitions	2.0	52.2	2,263.1	863.8				1,379.3
Net cash flows from contract and portfolio acquisition				2.5			(91.9)	(91.9)
Proceeds from Divestiture of Businesses					43			61
Net cash (used in) provided by financing activities								858.5
Collateral posted under agreement						160
Collateral posted under agreement as reduced								$ 100
Percentage of collateral under agreement to previous collateral requirement (as a percent)								25.00%