Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 825		$ 750
Ameren Missouri [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	16		66
Ameren Illinois Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1		84
Genco [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2		4
Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	186	[1]	274	[1]
Affiliates [Member] | Ameren Missouri [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			1	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	5		35
Coal Producers [Member] | Ameren Missouri [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			35
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	42		88
Commodity Marketing Companies [Member] | Ameren Missouri [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2		1
Commodity Marketing Companies [Member] | Ameren Illinois Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1		84
Commodity Marketing Companies [Member] | Genco [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1
Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	5		9
Electric Utilities [Member] | Ameren Missouri [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2		3
Electric Utilities [Member] | Genco [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			1
Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	21		65
Financial Companies [Member] | Ameren Missouri [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	8		22
Financial Companies [Member] | Genco [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			1
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	463		191
Municipalities/Cooperatives [Member] | Ameren Missouri [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	4		4
Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1		2
Oil And Gas Companies [Member] | Genco [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1		2
Retail Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 102		$ 86

[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.