Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended	9 Months Ended			3 Months Ended	9 Months Ended																													3 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 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	Sep. 30, 2011 Commodity contract	Sep. 30, 2011 Commodity contract	Sep. 30, 2010 Commodity contract	Sep. 30, 2011 Commodity contract Derivatives Designated as Hedging Instruments for Accounting Purposes	Sep. 30, 2010 Commodity contract Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Commodity contract Derivatives Designated as Hedging Instruments for Accounting Purposes	Sep. 30, 2011 Power contracts	Dec. 31, 2010 Power contracts	Sep. 30, 2011 Power contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Power contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Sep. 30, 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	Sep. 30, 2011 Gas contracts	Dec. 31, 2010 Gas contracts	Sep. 30, 2011 Gas contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Gas contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Sep. 30, 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	Sep. 30, 2011 Coal contracts	Dec. 31, 2010 Coal contracts	Sep. 30, 2011 Coal contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Coal contracts Derivatives Designated as Hedging Instruments for Accounting Purposes	Sep. 30, 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	Sep. 30, 2011 Other commodity contracts	Dec. 31, 2010 Other commodity contracts	Sep. 30, 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	Sep. 30, 2011 Interest rate contracts	Dec. 31, 2010 Interest rate contracts	Sep. 30, 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	Sep. 30, 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	Sep. 30, 2011 Foreign exchange contracts	Dec. 31, 2010 Foreign exchange contracts	Sep. 30, 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	Sep. 30, 2011 Equity contracts	Sep. 30, 2011 Equity contracts Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Sep. 30, 2011 Derivatives Designated as Hedging Instruments for Accounting Purposes	Dec. 31, 2010 Derivatives Designated as Hedging Instruments for Accounting Purposes	Sep. 30, 2011 Derivatives Not Designated As Hedging Instruments for Accounting Purposes	Dec. 31, 2010 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					$ (406.8)		$ (388.0)
Cash flow hedge gain (loss) to be reclassified within twelve months						0.1						224.7
Pre-tax gains (losses) recognized in earnings	(15.9)	(216.3)	(154.7)	(676.4)					(3.4)	(3.7)	(0.3)		(0.3)
Contract amount of interest rate swap						30.6
Average interest rate on interest rate swap						3.60%
Unrealized pre-tax loss, fair value of interest rate swap cash flow hedge																																							3.3
Notional amount, interest rate swap fair value hedge						550	400
Unrealized gain (loss), fair value of interest rate swap fair value hedge						47.0	35.7
Notional amount, interest rate swap not qualifying as fair value hedges								150
Asset values, all derivatives combined	10,258.2		10,258.2		13,760.8										4,645.3	7,962.9	617.0	1,167.9	4,028.3	6,795.0	4,817.1	5,292.4	1,270.0	1,902.3	3,547.1	3,390.1	137.4	363.0	28.4	97.0	109.0	266.0	538.9	61.4	538.9	61.4	104.7	70.1	47.0	35.7	57.7	34.4	14.4	11.0	14.4	11.0	0.4	0.4	1,962.4	3,202.9	8,295.8	10,557.9
Liability values, all derivatives combined	(10,612.8)		(10,612.8)		(13,931.4)										(5,006.2)	(8,529.3)	(683.7)	(1,362.8)	(4,322.5)	(7,166.5)	(4,897.6)	(4,988.1)	(1,302.8)	(1,832.8)	(3,594.8)	(3,155.3)	(119.1)	(308.3)	(22.3)	(48.6)	(96.8)	(259.7)	(529.2)	(61.6)	(529.2)	(61.6)	(55.2)	(35.7)	(3.3)		(51.9)	(35.7)	(5.5)	(8.4)	(5.5)	(8.4)			(2,012.1)	(3,244.2)	(8,600.7)	(10,687.2)
Netting arrangements, assets	(9,883.4)		(9,883.4)		(12,955.5)
Netting arrangements, liabilities	9,883.4		9,883.4		12,955.5
Cash collateral, assets	(81.4)		(81.4)		(28.4)
Cash collateral, liabilities	1.9		1.9		0.6
Net fair values, derivative assets	293.4		293.4		776.9
Net fair values, derivative liabilities	(727.5)		(727.5)		(975.3)
Derivatives classified as accounts receivable	(198.6)		(198.6)		(16.4)
Derivative assets, current	233.8		233.8		534.4
Derivative liabilities, current	(487.8)		(487.8)		(622.3)
Derivative assets, noncurrent	258.2		258.2		258.9
Derivative liabilities, noncurrent	$ (239.7)		$ (239.7)		$ (353.0)