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Retirement Benefits (Summary Of Estimated Amortizable Amounts From Regulatory Assets and Accumulated OCI Into Net Periodic Benefit Cost) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
Prior service credit $ (1)aee_DefinedBenefitPlanAmountsThatWillBeAmortizedFromRegulatoryAssetsOfNetPriorServiceCostCredit
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
[1]
Net actuarial loss 86aee_DefinedBenefitPlanAmountsThatWillBeAmortizedFromRegulatoryAssetsOfNetGainsLosses
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
[1]
Net actuarial (gain) loss 2us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
[1]
Net periodic benefit cost 87aee_PensionAndOtherPostretirementBenefitPlansAmountsThatWillBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossAndRegulatoryAssetsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
[1]
Postretirement Benefits  
Defined Benefit Plan Disclosure [Line Items]  
Prior service credit (4)aee_DefinedBenefitPlanAmountsThatWillBeAmortizedFromRegulatoryAssetsOfNetPriorServiceCostCredit
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
[1]
Net actuarial loss 15aee_DefinedBenefitPlanAmountsThatWillBeAmortizedFromRegulatoryAssetsOfNetGainsLosses
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
[1]
Net actuarial (gain) loss (2)us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
[1]
Net periodic benefit cost $ 9aee_PensionAndOtherPostretirementBenefitPlansAmountsThatWillBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossAndRegulatoryAssetsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
[1]
[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.