Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 138		$ 133
Current losses deferred as regulatory assets	247		215
Power [Member]
Derivative [Line Items]
Gain (loss) to be amortized in next year	14.0		5.0
Current gains deferred as regulatory liabilities	34		29
Current losses deferred as regulatory assets	32		17
Fuel Oils [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	20		16
Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1		1
Current losses deferred as regulatory assets	115		101
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	8		9
Accumulated Other Comprehensive Income (Loss) [Member] | Power [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	41	[1]	19	[1]
Accumulated Other Comprehensive Income (Loss) [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(8)	[2],[3]	(8)	[2],[3]
Regulatory Liabilities Or Assets [Member] | Power [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(81)	[4]	81	[4]
Regulatory Liabilities Or Assets [Member] | Fuel Oils [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	24	[5]	19	[5]
Regulatory Liabilities Or Assets [Member] | Natural Gas [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(209)	[6]	(191)	[6]
Regulatory Liabilities Or Assets [Member] | Uranium [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ (1)	[7]	$ (1)	[7]

[1] Represents net gains associated with power derivative contracts at Ameren. These contracts are a partial hedge of electricity price exposure through December 2016 as of March 31, 2012. Current gains of $14 million and $5 million were recorded at Ameren as of March 31, 2012, and December 31, 2011, respectively.

[2] 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 March 31, 2012, and December 31, 2011, was less than $1 million and less than $1 million, respectively. The balance of the gain will be amortized by June 2012.

[3] 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 March 31, 2012, and December 31, 2011, was a loss of $8 million and $9 million, respectively. Over the next twelve months, $1.4 million of the loss will be amortized.

[4] Represents net losses 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 2015 at Ameren Missouri, in each case as of March 31, 2012. Current gains deferred as regulatory liabilities include $34 million and $34 million at Ameren and Ameren Missouri, respectively, as of March 31, 2012. Current losses deferred as regulatory assets include $32 million, $13 million, and $203 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of March 31, 2012. Current gains deferred as regulatory liabilities include $29 million and $29 million at Ameren and Ameren Missouri, respectively, as of December 31, 2011. Current losses deferred as regulatory assets include $17 million, $8 million, and $209 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 31, 2011.

[5] Represents net gains on fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri's transportation costs for coal through October 2014 as of March 31, 2012. Current gains deferred as regulatory liabilities include $20 million and $20 million at Ameren and Ameren Missouri as of March 31, 2012, respectively. Current losses deferred as regulatory assets include less than $1 million and less than $1 million at Ameren and Ameren Missouri as of March 31, 2012, respectively. Current gains deferred as regulatory liabilities include $16 million and $16 million at Ameren and Ameren Missouri as of December 31, 2011, respectively. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri as of December 31, 2011, respectively.

[6] 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 March 31, 2012. Current gains deferred as regulatory liabilities include $1 million and $1 million at Ameren and Ameren Illinois, respectively, as of March 31, 2012. Current losses deferred as regulatory assets include $115 million, $13 million, and $102 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of March 31, 2012. Current gains deferred as regulatory liabilities include $1 million and $1 million at Ameren and Ameren Illinois, respectively, as of December 31, 2011. Current losses deferred as regulatory assets include $101 million, $11 million, and $90 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 31, 2011.

[7] Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of our uranium requirements through December 2013 as of March 31, 2012. Current losses deferred as regulatory assets include less than $1 million and less than $1 million at Ameren and Ameren Missouri as of March 31, 2012, respectively. Current losses deferred as regulatory assets include less than $1 million and less than $1 million at Ameren and Ameren Missouri as of December 31, 2011, respectively.