Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Power [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ 45	[1]	$ 19	[1]
Power [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(82)	[2]	81	[2]
Power [Member] | Ameren Missouri [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	24	[2]	21	[2]
Power [Member] | Ameren Illinois Company [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(221)	[2]	(140)	[2]
Fuel Oils [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	5	[3]	19	[3]
Fuel Oils [Member] | Ameren Missouri [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	5	[3]	19	[3]
Natural Gas [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(163)	[4]	(191)	[4]
Natural Gas [Member] | Ameren Missouri [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(21)	[4]	(24)	[4]
Natural Gas [Member] | Ameren Illinois Company [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(142)	[4]	(167)	[4]
Uranium [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[5]	(1)	[5]
Uranium [Member] | Ameren Missouri [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[5]	(1)	[5]
Interest Rate Contract [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(8)	[6],[7]	(8)	[6],[7]
Interest Rate Contract [Member] | Genco [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ (8)	[6],[7]	$ (8)	[6],[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 June 30, 2012. Current gains of $17 million and $5 million were recorded at Ameren as of June 30, 2012, and December 31, 2011, respectively.

[2] 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 June 30, 2012. Current gains deferred as regulatory liabilities include $37 million and $37 million at Ameren and Ameren Missouri, respectively, as of June 30, 2012. Current losses deferred as regulatory assets include $33 million, $14 million, and $133 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 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.

[3] 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 June 30, 2012. Current gains deferred as regulatory liabilities include $7 million and $7 million at Ameren and Ameren Missouri as of June 30, 2012, respectively. Current losses deferred as regulatory assets include $2 million and $2 million at Ameren and Ameren Missouri as of June 30, 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.

[4] 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 June 30, 2012. Current gains deferred as regulatory liabilities include $2 million and $2 million at Ameren and Ameren Illinois, respectively, as of June 30, 2012. Current losses deferred as regulatory assets include $88 million, $10 million, and $78 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 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.

[5] 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 June 30, 2012. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri as of June 30, 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.

[6] 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 June 30, 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.

[7] 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 covered the first 10 years of debt that has a 30-year maturity, and the gain in OCI was amortized over a 10-year period that began in June 2002. The balance of the gain was fully amortized as of June 30, 2012. The carrying value at December 31, 2011, was less than $1 million.