Derivative Instruments (Details 2) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	$ (73.6)	$ 114.9	$ (64.9)	$ (257.6)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(45.4)	(286.4)	(139.2)	(460.1)
Cash flow hedges gains (losses) related to hedge ineffectiveness	(5.9)	(37.6)	(22.8)	(24.3)
Fair value hedges gains (losses) related to hedge ineffectiveness	(1.9)		(1.8)
Derivative instruments designated in hedging relationships, gains (losses) related to hedge ineffectiveness	(7.8)	(37.6)	(24.6)	(24.3)
Pre-tax gain related to fair value hedges not included in hedge ineffectiveness	1.1		1.1
Amount of gain (loss) recognized in income on derivative	(15.2)	4.2	17.2	17.4
Amount of gain (loss) recognized in income on hedged item	(16.0)	(4.1)	(17.5)	(15.2)
Amount of gain (loss) recorded in income on derivative	48.3	47.1	44.9	10.9
Commodity contract
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(0.3)	1.1	(0.3)	(0.3)
Amount of gain (loss) recognized in income on derivative	0.8		0.8
Power contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(32.4)	(80.8)	(55.8)	121.7
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	23.3	(1.0)	5.0	(60.2)
Cash flow hedges gains (losses) related to hedge ineffectiveness	56.2	(20.5)	57.4	1.3
Power contracts | Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(14.9)	163.9	2.6	(291.6)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(133.3)	(331.6)	(232.1)	(534.7)
Cash flow hedges gains (losses) related to hedge ineffectiveness	(6.8)	(3.4)	(19.1)	(12.7)
Power contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	15.9	40.1	39.2	(24.8)
Gas contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	49.7	1.8	60.8	(33.1)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	73.1	16.2	95.3	36.4
Cash flow hedges gains (losses) related to hedge ineffectiveness	(51.2)	(2.9)	(49.0)	(4.0)
Gas contracts | Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(67.2)	(2.6)	(69.1)	(76.2)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(10.6)	45.1	(17.9)	123.1
Cash flow hedges gains (losses) related to hedge ineffectiveness	(5.2)	(13.1)	(12.9)	(13.1)
Gas contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	8.5	5.0	19.7	30.7
Coal contracts | Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(8.8)	32.6	(3.4)	21.8
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	2.1	(15.3)	9.4	(27.8)
Cash flow hedges gains (losses) related to hedge ineffectiveness	1.1	2.3	0.8	4.0
Coal contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	7.9	7.6	(0.7)	7.7
Other commodity contracts | Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings				(0.7)
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)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings				(0.3)
Cash flow hedges gains (losses) related to hedge ineffectiveness				0.2
Other commodity contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	7.3	(3.8)	(9.3)	1.1
Interest rate contracts | Hedges of interest rates: | Interest expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		0.2	1.1	4.1
Interest rate contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	0.1	(0.6)	(0.6)	(1.7)
Interest rate contracts | Interest expense
Derivative Instruments Gain (Loss)
Amount of gain (loss) recognized in income on derivative	14.4	4.2	16.4	17.4
Amount of gain (loss) recognized in income on hedged item	(16.0)	(4.1)	(17.5)	(15.2)
Amount of gain (loss) recorded in income on derivative	3.3		3.3
Foreign exchange contracts | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	5.3	(1.2)	(6.7)	(2.1)
Hedges of forecasted sales: | Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	17.3	(79.0)	5.0	88.6
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	96.4	15.2	100.3	(24.5)
Cash flow hedges gains (losses) related to hedge ineffectiveness	5.0	(23.4)	8.4	(2.7)
Hedges of forecasted purchases: | Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(90.9)	193.9	(69.9)	(346.2)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(141.8)	(301.8)	(240.6)	(439.7)
Cash flow hedges gains (losses) related to hedge ineffectiveness	(10.9)	(14.2)	(31.2)	(21.6)
Hedges of interest rates: | Interest expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		$ 0.2	$ 1.1	$ 4.1