Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Details) (USD $) In Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Derivative [Line Items]
Current losses deferred as regulatory assets	$ 184		$ 267
Current gains deferred as regulatory liabilities	123		99
Accumulated Other Comprehensive Income (Loss) [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(9)	[1],[2]	(9)	[1],[2]
Accumulated Other Comprehensive Income (Loss) [Member] | Power [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	3	[3]	8	[3]
Regulatory Liabilities Or Assets [Member] | Heating Oil [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	15	[4]	19	[4]
Regulatory Liabilities Or Assets [Member] | Natural Gas [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(142)	[5]	(165)	[5]
Regulatory Liabilities Or Assets [Member] | Power [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	104	[6]	1	[6]
Regulatory Liabilities Or Assets [Member] | Uranium [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[7]	2	[7]
Power [Member]
Derivative [Line Items]
Gain (loss) to be amortized in next year	3.0		8.0
Current losses deferred as regulatory assets	9		13
Current gains deferred as regulatory liabilities	24		8
Heating Oil [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	2		6
Current gains deferred as regulatory liabilities	14		13
Natural Gas [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	78		84
Current gains deferred as regulatory liabilities	2		1
Uranium [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Current gains deferred as regulatory liabilities			2
Interest Rate Swap [Member]
Derivative [Line Items]
Gain (loss) to be amortized in next year	(1.4)
Carrying value of net gains associated with interest rate swaps	1		1
Carrying value of net losses associated with interest rate swaps	$ 9		$ 10

[1] Includes net gains associated with interest rate swaps at Genco that were a partial hedge of the interest rate on debt issued in June 2002. The swaps cover the first 10 years of debt that has a 30-year maturity, and the gain in OCI is amortized over a 10-year period that began in June 2002. The carrying value at September 30, 2011, and December 31, 2010, was less than $1 million and less than $1 million, respectively. The balance of the gain will be amortized by June 2012.

[2] Includes net losses associated with interest rate swaps at Genco. The swaps were executed during the fourth quarter of 2007 as a partial hedge of interest rate risks associated with Genco's April 2008 debt issuance. The loss on the interest rate swaps is being amortized over a 10-year period that began in April 2008. The carrying value at September 30, 2011, and December 31, 2010, was a loss of $9 million and $10 million, respectively. Over the next twelve months, $1.4 million of the loss will be amortized.

[3] Represents net gains associated with power derivative contracts at Ameren. These contracts are a partial hedge of electricity price exposure through December 2013 as of September 30, 2011. Current gains of $3 million and $8 million were recorded at Ameren as of September 30, 2011, and December 31, 2010, respectively.

[4] Represents net gains on heating oil derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri's transportation costs for coal through December 2013 as of September 30, 2011. Current gains deferred as regulatory liabilities include $14 million and $14 million at Ameren and Ameren Missouri as of September 30, 2011, respectively. Current losses deferred as regulatory assets include $2 million and $2 million at Ameren and Ameren Missouri as of September 30, 2011, respectively. Current gains deferred as regulatory liabilities include $13 million and $13 million at Ameren and Ameren Missouri as of December 31, 2010, respectively. Current losses deferred as regulatory assets include $6 million and $6 million at Ameren and Ameren Missouri as of December 31, 2010, respectively.

[5] Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through October 2016 at Ameren, Ameren Missouri, and Ameren Illinois, in each case as of September 30, 2011. Current gains deferred as regulatory liabilities include $2 million and $2 million at Ameren and Ameren Illinois, respectively, as of September 30, 2011. Current losses deferred as regulatory assets include $78 million, $9 million, and $69 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of September 30, 2011. Current gains deferred as regulatory liabilities include $1 million and $1 million at Ameren and Ameren Missouri, respectively, as of December 31, 2010. Current losses deferred as regulatory assets include $84 million, $11 million, and $73 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 31, 2010.

[6] Represents net gains associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2012 at Ameren Missouri, in each case as of September 30, 2011. Current gains deferred as regulatory liabilities include $24 million, $23 million, and $1 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of September 30, 2011. Current losses deferred as regulatory assets include $9 million, $4 million, and $170 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of September 30, 2011. Current gains deferred as regulatory liabilities include $8 million, $6 million, and $2 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 31, 2010. Current losses deferred as regulatory assets include $13 million, $3 million, and $181 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 31, 2010.

[7] Represents net losses and gains on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of our uranium requirements through December 2013 as of September 30, 2011. Current losses deferred as regulatory assets include less than $1 million and less than $1 million at Ameren and Ameren Missouri as of September 30, 2011, respectively. Current gains deferred as regulatory liabilities include $2 million and $2 million at Ameren and Ameren Missouri as of December 31, 2010, respectively.