Derivatives and Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended								12 Months Ended					12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Jan. 31, 2011 Interest Rate Swaps	Dec. 31, 2011 Interest Rate Swaps Derivatives Designated as Hedging Instruments for Accounting Purposes	Sep. 30, 2011 Interest Rate Swaps Derivatives Designated as Hedging Instruments for Accounting Purposes	Feb. 28, 2011 Interest Rate Swaps Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Interest Rate Swaps Derivatives Designated as Hedging Instruments for Accounting Purposes	Feb. 28, 2011 Interest Rate Swaps Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Commodity contract	Dec. 31, 2010 Commodity contract	Dec. 31, 2009 Commodity contract	Dec. 31, 2011 Commodity contract Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Commodity contract Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Power contracts	Dec. 31, 2010 Power contracts	Dec. 31, 2011 Power contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Power contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Power contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Power contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Gas contracts	Dec. 31, 2010 Gas contracts	Dec. 31, 2011 Gas contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Gas contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Gas contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Gas contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Coal contracts	Dec. 31, 2010 Coal contracts	Dec. 31, 2011 Coal contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Coal contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Coal contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Coal contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Other commodity contracts	Dec. 31, 2010 Other commodity contracts	Dec. 31, 2011 Other commodity contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Other commodity contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Interest rate contracts	Dec. 31, 2010 Interest rate contracts	Dec. 31, 2011 Interest rate contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Interest rate contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Interest rate contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Interest rate contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Foreign exchange contracts	Dec. 31, 2010 Foreign exchange contracts	Dec. 31, 2011 Foreign exchange contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Foreign exchange contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2011 Equity contracts	Dec. 31, 2011 Equity contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes
Derivatives, Fair Value
Net unrealized pre-tax (gain) loss on cash-flow hedges recorded in Accumulated other comprehensive loss									$ (8.9)			$ (10.1)					$ 652.7	$ 388.0
Cash flow hedge gain (loss) to be reclassified within twelve months									0.1								392.6
Pre-tax gains (losses) recognized in earnings	(287.2)	(926.9)	(2,397.9)											(4.0)	(0.3)	(241.0)
Contract amount of interest rate swap										30.6
Average interest rate on interest rate swap (as a percent)										3.60%
Unrealized pre-tax loss, fair value of interest rate swap cash flow hedge									(3.6)
Notional amount, interest rate swap fair value hedge									550		350	400	150
Unrealized gain (loss), fair value of interest rate swap fair value hedge									43.8			35.7
Portion of debt related to terminated interest rate swap fair value hedge								200
Proceeds from terminated interest rate swap fair value hedge	13.8
Asset values, all derivatives combined	13,240.8	13,760.8		3,319.6	3,202.9	9,921.2	10,557.9												6,403.0	7,962.9	1,617.2	1,167.9	4,785.8	6,795.0	6,467.3	5,292.4	1,624.5	1,902.3	4,842.8	3,390.1	107.0	363.0	34.0	97.0	73.0	266.0	153.8	61.4	153.8	61.4	99.0	70.1	43.9	35.7	55.1	34.4	10.5	11.0	10.5	11.0	0.2	0.2
Liability values, all derivatives combined	(14,061.2)	(13,931.4)		(3,834.9)	(3,244.2)	(10,226.3)	(10,687.2)												(6,792.8)	(8,529.3)	(1,686.9)	(1,362.8)	(5,105.9)	(7,166.5)	(6,966.5)	(4,988.1)	(2,108.4)	(1,832.8)	(4,858.1)	(3,155.3)	(102.3)	(308.3)	(36.0)	(48.6)	(66.3)	(259.7)	(144.2)	(61.6)	(144.2)	(61.6)	(52.6)	(35.7)	(3.6)		(49.0)	(35.7)	(2.8)	(8.4)	(2.8)	(8.4)
Netting arrangements, assets	(12,989.5)	(12,955.5)
Netting arrangements, liabilities	12,989.5	12,955.5
Cash collateral, assets	(167.2)	(28.4)
Cash collateral, liabilities	23.8	0.6
Net fair values, derivative assets	84.1	776.9
Net fair values, derivative liabilities	(1,047.9)	(975.3)
Derivatives classified as accounts receivable	(533.1)	(16.4)
Derivative assets, current	357.9	534.4
Derivative liabilities, current	(779.5)	(622.3)
Derivative assets, noncurrent	259.3	258.9
Derivative liabilities, noncurrent	$ (268.4)	$ (353.0)