Derivative Instruments (Details 2) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	$ (106.6)	$ (238.6)	$ (171.3)	$ (496.2)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(15.9)	(216.3)	(154.7)	(676.4)
Cash flow hedges gains (losses) related to hedge ineffectiveness	(76.8)	(21.8)	(99.6)	(46.1)
Fair value hedges gains (losses) related to hedge ineffectiveness	0.6		(1.2)
Derivative instruments designated in hedging relationships, gains (losses) related to hedge ineffectiveness	(76.2)	(21.8)	(100.8)	(46.1)
Pre-tax gain related to fair value hedges not included in hedge ineffectiveness	1.6		2.7
Amount of gain (loss) recognized in income on derivative	22.4	14.2	39.6	31.6
Amount of gain (loss) recognized in income on hedged item	(20.2)	(14.1)	(37.7)	(29.3)
Amount of gain (loss) recorded in income on derivative	(25.6)	(23.3)	18.5	(12.4)
Commodity contract
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	0
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(3.4)		(3.7)	(0.3)
Power contracts \| Hedges of forecasted sales: \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	25.0	92.1	(30.8)	213.8
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(7.7)	(72.5)	2.2	(132.7)
Cash flow hedges gains (losses) related to hedge ineffectiveness	0.6	14.3	58.1	15.6
Power contracts \| Hedges of forecasted purchases: \| Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(16.9)	(211.6)	(14.3)	(503.2)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(63.5)	(195.3)	(297.0)	(730.0)
Cash flow hedges gains (losses) related to hedge ineffectiveness	(13.6)	(16.3)	(32.7)	(29.0)
Power contracts \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	23.2	(2.5)	63.3	(27.3)
Gas contracts \| Hedges of forecasted sales: \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(2.9)	(20.2)	58.2	(53.3)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	48.0	19.4	128.5	55.8
Cash flow hedges gains (losses) related to hedge ineffectiveness	4.4	(0.2)	(44.7)	(4.2)
Gas contracts \| Hedges of forecasted purchases: \| Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(110.8)	(119.7)	(180.0)	(195.9)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(4.3)	41.1	(10.5)	164.2
Cash flow hedges gains (losses) related to hedge ineffectiveness	(69.7)	(20.0)	(82.6)	(33.1)
Gas contracts \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recognized in income on derivative	6.6		7.4
Amount of gain (loss) recognized in income on hedged item	(6.2)		(6.2)
Amount of gain (loss) recorded in income on derivative	(57.9)	(12.8)	(39.8)	17.9
Coal contracts \| Hedges of forecasted purchases: \| Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(4.3)	20.8	(7.7)	42.6
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	11.6	(9.1)	21.0	(36.9)
Cash flow hedges gains (losses) related to hedge ineffectiveness	1.5	0.4	2.3	4.4
Coal contracts \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	(6.7)	1.7	(7.4)	9.4
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	3.2	(8.9)	(6.1)	(7.8)
Interest rate contracts \| Hedges of interest rates: \| Interest expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	3.3		3.3
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		0.1	1.1	4.2
Interest rate contracts \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	0.9	(1.9)	0.3	(3.6)
Interest rate contracts \| Interest expense
Derivative Instruments Gain (Loss)
Amount of gain (loss) recognized in income on derivative	15.8	14.2	32.2	31.6
Amount of gain (loss) recognized in income on hedged item	(14.0)	(14.1)	(31.5)	(29.3)
Amount of gain (loss) recorded in income on derivative	2.5		5.8
Foreign exchange contracts \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	9.5	1.1	2.7	(1.0)
Equity contracts \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Amount of gain (loss) recorded in income on derivative	(0.3)		(0.3)
Hedges of forecasted sales: \| Nonregulated Revenues
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	22.1	71.9	27.4	160.5
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	40.3	(53.1)	130.7	(77.6)
Cash flow hedges gains (losses) related to hedge ineffectiveness	5.0	14.1	13.4	11.4
Hedges of forecasted purchases: \| Fuel and purchased energy expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	(132.0)	(310.5)	(202.0)	(656.7)
Cash flow hedges gain (loss) reclassified from AOCI into Earnings	(56.2)	(163.3)	(286.5)	(603.0)
Cash flow hedges gains (losses) related to hedge ineffectiveness	(81.8)	(35.9)	(113.0)	(57.5)
Hedges of interest rates: \| Interest expense
Derivative Instruments Gain (Loss)
Cash flow hedges gain (loss) recorded in AOCI	3.3		3.3
Cash flow hedges gain (loss) reclassified from AOCI into Earnings		$ 0.1	$ 1.1	$ 4.2

The amount of net gains or losses on a cash flow hedge reclassified in the period to earnings from accumulated other comprehensive income.

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The amount recognized in earnings related to the change in fair value during the period of derivative or nonderivative instruments designated as fair value hedging instruments.

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The amount recognized in earnings related to the change in fair value during the period of the hedged item in a fair value hedge.

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Net gain (loss) from the increase (decrease) in fair value of derivative instrument not designated as hedging instrument.

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The portion of a hedge deemed to be ineffective that is reflected in current period results of operations.

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The net gain (loss) recognized in earnings during the reporting period representing the amount of the cash flow hedges' ineffectiveness.

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Net gain (loss) recognized in earnings during the reporting period representing the amount of the fair value of the hedges' ineffectiveness.

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Amount of unrealized gain (loss) related to derivatives designated as cash flow hedging instruments, recorded in results of operations to the extent that the cash flow hedge is determined to be ineffective.

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