Fair Value Measurements (Details) (USD $) In Millions	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Assets and liabilities recognized at fair value
Cash equivalents	$ 316.6	$ 1,545.4
Equity securities-mutual funds	44.2	43.7
Derivative assets, current	324.3	534.4
Derivative assets, noncurrent	257.5	258.9
Derivative liabilities, current	(494.2)	(622.3)
Derivative liabilities, noncurrent	(270.1)	(353.0)
Derivative instruments classified as derivative assets	581.8	793.3
Derivative instruments classified as derivative liabilities	(764.3)	(975.3)
Derivatives classified as accounts receivable	(72.7)	(16.4)
Derivative assets, net	509.1	776.9
Derivative assets, gross	10,285.8	13,760.8
Net fair values, derivative liabilities	(764.3)	(975.3)
Derivative liabilities, gross	(10,484.7)	(13,931.4)
Assets measured at fair value on a recurring basis	869.9	2,366.0
Liabilities measured at fair value on a recurring basis	(764.3)	(975.3)
Cash collateral held	58.8	28.4
Cash collateral posted	2.5	0.6
Gas contracts \| Level 1
Assets and liabilities recognized at fair value
Derivative assets, gross	68.9	63.9
Derivative liabilities, gross	(66.5)	(72.7)
Other commodity contracts \| Level 1
Assets and liabilities recognized at fair value
Derivative assets, gross	50.7	6.6
Derivative liabilities, gross	(48.7)	(7.1)
Interest rate contracts \| Level 1
Assets and liabilities recognized at fair value
Derivative assets, gross	35.8	33.1
Derivative liabilities, gross	(37.9)	(35.7)
Level 1
Assets and liabilities recognized at fair value
Cash equivalents	316.6	1,545.4
Equity securities-mutual funds	44.2	43.7
Derivative assets, gross	155.4	103.6
Derivative liabilities, gross	(153.1)	(115.5)
Net derivative position	2.3	(11.9)
Assets and liabilities, net, measured at fair value on a recurring basis	363.1	1,577.2
Power contracts \| Level 2
Assets and liabilities recognized at fair value
Derivative assets, gross	4,718.2	7,509.6
Derivative liabilities, gross	(5,004.4)	(7,758.2)
Gas contracts \| Level 2
Assets and liabilities recognized at fair value
Derivative assets, gross	3,568.9	5,113.3
Derivative liabilities, gross	(3,530.6)	(4,910.3)
Coal contracts \| Level 2
Assets and liabilities recognized at fair value
Derivative assets, gross	209.1	355.6
Derivative liabilities, gross	(174.8)	(307.4)
Other commodity contracts \| Level 2
Assets and liabilities recognized at fair value
Derivative assets, gross	58.6	54.8
Derivative liabilities, gross	(51.8)	(54.5)
Interest rate contracts \| Level 2
Assets and liabilities recognized at fair value
Derivative assets, gross	40.2	37.0
Foreign exchange contracts \| Level 2
Assets and liabilities recognized at fair value
Derivative assets, gross	13.7	11.0
Derivative liabilities, gross	(8.2)	(8.4)
Level 2
Assets and liabilities recognized at fair value
Derivative assets, gross	8,608.7	13,081.3
Derivative liabilities, gross	(8,769.8)	(13,038.8)
Net derivative position	(161.1)	42.5
Assets and liabilities, net, measured at fair value on a recurring basis	(161.1)	42.5
Power contracts \| Level 3
Assets and liabilities recognized at fair value
Derivative assets, gross	873.5	453.3
Derivative liabilities, gross	(1,011.0)	(771.1)
Gas contracts \| Level 3
Assets and liabilities recognized at fair value
Derivative assets, gross	451.0	115.2
Derivative liabilities, gross	(351.5)	(5.1)
Coal contracts \| Level 3
Assets and liabilities recognized at fair value
Derivative assets, gross	0.6	7.4
Derivative liabilities, gross		(0.9)
Other commodity contracts \| Level 3
Assets and liabilities recognized at fair value
Derivative assets, gross	196.6
Derivative liabilities, gross	(199.3)
Level 3
Assets and liabilities recognized at fair value
Derivative assets, gross	1,521.7	575.9
Derivative liabilities, gross	(1,561.8)	(777.1)
Net derivative position	(40.1)	(201.2)
Assets and liabilities, net, measured at fair value on a recurring basis	(40.1)	(201.2)
Netting and Cash Collateral
Assets and liabilities recognized at fair value
Derivative assets, gross	(9,776.7)	(12,983.9)
Derivative liabilities, gross	9,720.4	12,956.1
Net derivative position	(56.3)	(27.8)
Assets and liabilities, net, measured at fair value on a recurring basis	(56.3)	(27.8)
Fair Value
Assets and liabilities recognized at fair value
Cash equivalents	316.6	1,545.4
Equity securities-mutual funds	44.2	43.7
Derivative assets, gross	509.1	776.9
Derivative liabilities, gross	(764.3)	(975.3)
Net derivative position	(255.2)	(198.4)
Assets and liabilities, net, measured at fair value on a recurring basis	105.6	1,390.7
Power contracts
Assets and liabilities recognized at fair value
Derivative assets, gross	5,591.7	7,962.9
Derivative liabilities, gross	(6,015.4)	(8,529.3)
Gas contracts
Assets and liabilities recognized at fair value
Derivative assets, gross	4,088.8	5,292.4
Derivative liabilities, gross	(3,948.6)	(4,988.1)
Coal contracts
Assets and liabilities recognized at fair value
Derivative assets, gross	209.7	363.0
Derivative liabilities, gross	(174.8)	(308.3)
Other commodity contracts
Assets and liabilities recognized at fair value
Derivative assets, gross	305.9	61.4
Derivative liabilities, gross	(299.8)	(61.6)
Interest rate contracts
Assets and liabilities recognized at fair value
Derivative assets, gross	76.0	70.1
Derivative liabilities, gross	(37.9)	(35.7)
Foreign exchange contracts
Assets and liabilities recognized at fair value
Derivative assets, gross	13.7	11.0
Derivative liabilities, gross	$ (8.2)	$ (8.4)

The aggregate of current and noncurrent derivative instruments that are measured at fair value on a recurring basis, and are classified on the balance sheet as derivative assets.

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The aggregate of current and noncurrent derivative instruments that are measured at fair value on a recurring basis, and are classified on the balance sheet as derivative liabilities.

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This element represents the investment that pools funds from many investors to invest in a combination of underlying investments, primarily mutual funds.

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Represents the unrealized fair value of exchange traded derivatives, exclusive of cash margin posted.

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The carrying amounts of assets and liabilities recognized at fair values.

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Represents the net amount of assets and liabilities measured at fair value on a recurring basis.

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Represents the amount of assets measured at fair value on a recurring basis.

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Represents the amount of liabilities measured at fair value on a recurring basis.

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The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

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Indicates, if estimable (as a monetary amount), the approximate extent to which the proceeds from liquidation of any assets held either as collateral or by third parties would be expected to cover the maximum potential amount of future payments under the credit derivative or each group of similar credit derivatives.

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Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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Fair values as of the balance sheet date for all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments and which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer, net of the effects of master netting arrangements.

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Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments which are expected to exist longer than one year or beyond the normal operating cycle, if longer, net of the effects of master netting arrangements.

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Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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Fair value as of the balance sheet date of the assets less the liabilities of a derivative or group of derivatives.

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Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be extinguished or otherwise disposed of within a year or the normal operating cycle, if longer, net of the effects of master netting arrangements.

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Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be extinguished or otherwise disposed of after one year or beyond the normal operating cycle, if longer, net of the effects of master netting arrangements.

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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