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Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Derivative [Line Items]  
Aggregate Fair Value of Derivative Liabilities $ 167us-gaap_DerivativeNetLiabilityPositionAggregateFairValue [1]
Cash Collateral Posted 8us-gaap_CollateralAlreadyPostedAggregateFairValue
Potential Aggregate Amount of Additional Collateral Required 157us-gaap_AdditionalCollateralAggregateFairValue [2]
Union Electric Company  
Derivative [Line Items]  
Aggregate Fair Value of Derivative Liabilities 87us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 [1]
Cash Collateral Posted 6us-gaap_CollateralAlreadyPostedAggregateFairValue
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Potential Aggregate Amount of Additional Collateral Required 83us-gaap_AdditionalCollateralAggregateFairValue
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 [2]
Ameren Illinois Company  
Derivative [Line Items]  
Aggregate Fair Value of Derivative Liabilities 80us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 [1]
Cash Collateral Posted 2us-gaap_CollateralAlreadyPostedAggregateFairValue
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Potential Aggregate Amount of Additional Collateral Required $ 74us-gaap_AdditionalCollateralAggregateFairValue
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 [2]
[1] Prior to consideration of master netting arrangements and including NPNS and other accrual contract exposures.
[2] As collateral requirements with certain counterparties are based on master netting arrangements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.