Derivatives and Fair Value Measurements (Details 5) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fair value assets and liabilities measured on recurring basis
Derivative assets, current	$ 357.9	$ 534.4
Derivative assets, noncurrent	259.3	258.9
Derivative liabilities, current	(779.5)	(622.3)
Derivative liabilities, noncurrent	(268.4)	(353.0)
Derivatives classified as accounts receivable	(533.1)	(16.4)
Derivative assets, net	84.1	776.9
Derivative assets, gross	13,240.8	13,760.8
Net fair values, derivative liabilities	(1,047.9)	(975.3)
Derivative liabilities, gross	(14,061.2)	(13,931.4)
Cash collateral held	167.2	28.4
Cash collateral posted	23.8	0.6
Power contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	6,403.0	7,962.9
Derivative liabilities, gross	(6,792.8)	(8,529.3)
Gas contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	6,467.3	5,292.4
Derivative liabilities, gross	(6,966.5)	(4,988.1)
Coal contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	107.0	363.0
Derivative liabilities, gross	(102.3)	(308.3)
Other commodity contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	153.8	61.4
Derivative liabilities, gross	(144.2)	(61.6)
Interest rate contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	99.0	70.1
Derivative liabilities, gross	(52.6)	(35.7)
Foreign exchange contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	10.5	11.0
Derivative liabilities, gross	(2.8)	(8.4)
Equity contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	0.2
Recurring Measurements
Fair value assets and liabilities measured on recurring basis
Cash equivalents	344.2	1,545.4
Debt and equity securities	39.8	47.8
Derivative assets, current	357.9	534.4
Derivative assets, noncurrent	259.3	258.9
Derivative liabilities, current	(779.5)	(622.3)
Derivative liabilities, noncurrent	(268.4)	(353.0)
Derivative instruments classified as derivative assets	617.2	793.3
Derivative instruments classified as derivative liabilities	(1,047.9)	(975.3)
Derivatives classified as accounts receivable	(533.1)	(16.4)
Derivative assets, net	84.1	776.9
Net fair values, derivative liabilities	(1,047.9)	(975.3)
Assets measured at fair value	468.1	2,370.1
Liabilities measured at fair value	(1,047.9)	(975.3)
Cash collateral held	167.2	28.4
Cash collateral posted	23.8	0.6
Recurring Measurements | Level 1
Fair value assets and liabilities measured on recurring basis
Cash equivalents	344.2	1,545.4
Debt and equity securities	30.5	43.7
Derivative assets, gross	480.7	103.6
Derivative liabilities, gross	(500.5)	(115.5)
Net derivative position	(19.8)	(11.9)
Assets and liabilities, net, measured at fair value	354.9	1,577.2
Recurring Measurements | Level 1 | Gas contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	430.5	63.9
Derivative liabilities, gross	(450.4)	(72.7)
Recurring Measurements | Level 1 | Other commodity contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	2.0	6.6
Derivative liabilities, gross	(2.2)	(7.1)
Recurring Measurements | Level 1 | Interest rate contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	48.2	33.1
Derivative liabilities, gross	(47.9)	(35.7)
Recurring Measurements | Level 2
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	11,409.7	13,081.3
Derivative liabilities, gross	(12,351.5)	(13,038.8)
Net derivative position	(941.8)	42.5
Assets and liabilities, net, measured at fair value	(941.8)	42.5
Recurring Measurements | Level 2 | Power contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	5,835.2	7,509.6
Derivative liabilities, gross	(6,175.0)	(7,758.2)
Recurring Measurements | Level 2 | Gas contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	5,371.2	5,113.3
Derivative liabilities, gross	(6,046.9)	(4,910.3)
Recurring Measurements | Level 2 | Coal contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	106.8	355.6
Derivative liabilities, gross	(101.6)	(307.4)
Recurring Measurements | Level 2 | Other commodity contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	35.2	54.8
Derivative liabilities, gross	(25.2)	(54.5)
Recurring Measurements | Level 2 | Interest rate contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	50.8	37.0
Recurring Measurements | Level 2 | Foreign exchange contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	10.5	11.0
Derivative liabilities, gross	(2.8)	(8.4)
Recurring Measurements | Level 3
Fair value assets and liabilities measured on recurring basis
Debt and equity securities	9.3	4.1
Derivative assets, gross	1,350.4	575.9
Derivative liabilities, gross	(1,209.2)	(777.1)
Net derivative position	141.2	(201.2)
Assets and liabilities, net, measured at fair value	150.5	(197.1)
Recurring Measurements | Level 3 | Power contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	567.8	453.3
Derivative liabilities, gross	(617.8)	(771.1)
Recurring Measurements | Level 3 | Gas contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	665.6	115.2
Derivative liabilities, gross	(470.3)	(5.1)
Recurring Measurements | Level 3 | Coal contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	0.2	7.4
Derivative liabilities, gross	(0.7)	(0.9)
Recurring Measurements | Level 3 | Other commodity contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	116.6
Derivative liabilities, gross	(116.8)
Recurring Measurements | Level 3 | Interest rate contracts
Fair value assets and liabilities measured on recurring basis
Derivative liabilities, gross	(3.6)
Recurring Measurements | Level 3 | Equity contracts
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	0.2
Recurring Measurements | Netting and Cash Collateral
Fair value assets and liabilities measured on recurring basis
Derivative assets, gross	(13,156.7)	(12,983.9)
Derivative liabilities, gross	13,013.3	12,956.1
Net derivative position	(143.4)	(27.8)
Assets and liabilities, net, measured at fair value	(143.4)	(27.8)
Recurring Measurements | Fair Value
Fair value assets and liabilities measured on recurring basis
Cash equivalents	344.2	1,545.4
Debt and equity securities	39.8	47.8
Derivative assets, gross	84.1	776.9
Derivative liabilities, gross	(1,047.9)	(975.3)
Net derivative position	(963.8)	(198.4)
Assets and liabilities, net, measured at fair value	$ (579.8)	$ 1,394.8