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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt (including current portion) $ 7,127us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1] $ 7,135us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1]
Preferred stock 123us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1] 122us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1]
Fair Value | Union Electric Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt (including current portion) 4,574us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
4,518us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Preferred stock 74us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
73us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Fair Value | Ameren Illinois Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt (including current portion) 2,553us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
2,517us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Preferred stock 49us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
49us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt (including current portion) 6,240us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
[1] 6,240us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
[1]
Preferred stock 142us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
[1] 142us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
[1]
Carrying Amount | Union Electric Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt (including current portion) 3,999us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
3,999us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Preferred stock 80us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
80us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Carrying Amount | Ameren Illinois Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt (including current portion) 2,241us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
2,241us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Preferred stock $ 62us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 62us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
[1] Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.