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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt and Capital Lease Obligations $ 6,240us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
[1] $ 6,038us-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]
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt and capital lease obligations (including current portion) 7,135us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1] 6,584us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1]
Preferred stock 122us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1] 118us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1]
Union Electric Company | Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt and Capital Lease Obligations 3,999us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
3,757us-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
Union Electric Company | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt and capital lease obligations (including current portion) 4,518us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
4,124us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Preferred stock 73us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
71us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company | Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt and Capital Lease Obligations 2,241us-gaap_LongTermDebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
1,856us-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
Ameren Illinois Company | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt and capital lease obligations (including current portion) 2,517us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
2,028us-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
$ 47us-gaap_PreferredStockValueOutstanding
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
[1] Preferred stock is recorded in "Noncontrolling Interests" on the consolidated balance sheet.