Derivatives and Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Jan. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		$ (556.7)	$ (367.8)	$ (725.9)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		(287.2)	(926.9)	(2,397.9)
Cash flow hedges gains (losses) related to hedge ineffectiveness		(132.4)	(91.3)	11.3
Fair value hedges gains (losses) related to hedge ineffectiveness		(0.7)		23.9
Derivative instruments designated in hedging relationships, gains (losses) related to hedge ineffectiveness		(133.1)	(91.3)	35.2
Pre-tax gain related to fair value hedges not included in hedge ineffectiveness		5.8
Amount of gain (loss) recognized in income on derivative		(56.3)	18.0	40.5
Amount of gain (loss) recognized in income on hedged item		(48.7)	(15.6)	(16.0)
Amount of gain (loss) recorded in income on derivative		(282.5)	9.6	(236.2)
Amount of terminated interest rate swaps	200
Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		225.8	85.4	316.4
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		217.7	(76.7)	(479.6)
Cash flow hedges gains (losses) related to hedge ineffectiveness		21.3	8.6	77.6
Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		(778.9)	(453.2)	(1,042.3)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		(506.1)	(854.5)	(1,918.9)
Cash flow hedges gains (losses) related to hedge ineffectiveness		(153.7)	(99.9)	(66.3)
Hedges of interest rates: | Interest expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		(3.6)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		1.2	4.3	0.6
Commodity contract
Derivative Instruments Gain (Loss)
Origination Gains recognized associated with derivative instruments		14.8
Number of transactions in which origination gains are recognized		1
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		(4.0)	(0.3)	(241.0)
Gas transportation capacity contract
Derivative Instruments Gain (Loss)
Number of contract years for mitigation of risk (in years)		2
Power contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative		(51.4)	(26.2)	250.9
Power contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		162.3	144.5	362.5
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		19.2	(165.8)	(180.6)
Cash flow hedges gains (losses) related to hedge ineffectiveness		70.9	8.9	77.5
Power contracts | Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		(295.5)	(377.4)	(1,056.0)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		(476.7)	(1,036.1)	(1,905.3)
Cash flow hedges gains (losses) related to hedge ineffectiveness		(52.0)	(40.7)	(42.2)
Gas contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recognized in income on derivative		23.4		40.6
Amount of gain (loss) recognized in income on hedged item		(15.9)		(16.7)
Amount of gain (loss) recorded in income on derivative		(224.3)	41.4	(360.0)
Gas contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		63.5	(59.1)	(65.1)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		198.5	90.8	(67.3)
Cash flow hedges gains (losses) related to hedge ineffectiveness		(49.6)	(0.3)	6.3
Gas contracts | Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		(471.6)	(141.5)	103.7
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		(51.8)	216.5	165.8
Cash flow hedges gains (losses) related to hedge ineffectiveness		(102.5)	(64.3)	(15.2)
Coal contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative		(9.2)	13.3	14.0
Coal contracts | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative				(109.8)
Coal contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI				10.0
Cash flow hedges gain (loss) reclassified from AOCI into Earnings				(229.9)
Coal contracts | Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		(11.8)	65.9	(77.7)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		22.4	(34.6)	(187.6)
Cash flow hedges gains (losses) related to hedge ineffectiveness		0.8	4.9	(8.9)
Other commodity contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative		(4.4)	(15.4)	(11.7)
Other commodity contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI				6.8
Cash flow hedges gain (loss) reclassified from AOCI into Earnings			0.7	(0.4)
Cash flow hedges gains (losses) related to hedge ineffectiveness				(6.2)
Other commodity contracts | Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI			(0.2)	(12.3)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings			(0.3)	8.2
Cash flow hedges gains (losses) related to hedge ineffectiveness			0.2
Interest rate contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative		1.6	(2.3)	(27.2)
Interest rate contracts | Interest expense
Derivative Instruments Gain (Loss)
Amount of gain (loss) recognized in income on derivative		32.9	18.0	(0.1)
Amount of gain (loss) recognized in income on hedged item		(32.8)	(15.6)	0.7
Amount of gain (loss) recorded in income on derivative		5.2
Interest rate contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI				(0.3)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings				(0.3)
Interest rate contracts | Hedges of interest rates: | Interest expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI		(3.6)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		1.2	4.3	0.6
Foreign exchange contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative		0.4	(1.2)	7.6
Foreign exchange contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI				2.5
Cash flow hedges gain (loss) reclassified from AOCI into Earnings			(1.0)	(1.1)
Equity contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative		$ (0.4)