Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 881		$ 790
Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	29		71
Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	2		85
Genco [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	6		13
Affiliates [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	188	[1]	276	[1]
Affiliates [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1	[1]	1	[1]
Coal Producers [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	11		37
Coal Producers [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1		35
Coal Producers [Member] | Genco [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts			1
Commodity Marketing Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	51		89
Commodity Marketing Companies [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	2		1
Commodity Marketing Companies [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1		84
Commodity Marketing Companies [Member] | Genco [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	2		1
Electric Utilities [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	20		16
Electric Utilities [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	6		4
Electric Utilities [Member] | Genco [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts			2
Financial Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	35		84
Financial Companies [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	15		26
Financial Companies [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1		1
Financial Companies [Member] | Genco [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1		6
Municipalities/Cooperatives [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	469		198
Municipalities/Cooperatives [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	4		4
Oil And Gas Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	4		3
Oil And Gas Companies [Member] | Genco [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	3		3
Retail Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 103		$ 87

[1] Primarily comprised of Marketing Company's exposure to Ameren Illinois related to financial contracts. The exposure is not eliminated at the consolidated Ameren level for purposes of this disclosure, as it is calculated without regard to the offsetting affiliate counterparty's liability position. See Note 14-Related Party Transactions in the Form 10-K for additional information on these financial contracts.