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Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Federal) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Nov. 30, 2014
Feb. 28, 2015
Sep. 30, 2014
Rate And Regulatory Matters [Line Items]            
Current regulatory liabilities $ 106us-gaap_RegulatoryLiabilityCurrent $ 216us-gaap_RegulatoryLiabilityCurrent        
Purchased power 454us-gaap_CostOfPurchasedPower 502us-gaap_CostOfPurchasedPower 780us-gaap_CostOfPurchasedPower      
Miscellaneous income 79us-gaap_OtherNonoperatingIncome [1] 69us-gaap_OtherNonoperatingIncome [1] 70us-gaap_OtherNonoperatingIncome [1]      
Midwest Independent Transmission System Operator, Inc [Member] | Pending Ferc Case [Member]            
Rate And Regulatory Matters [Line Items]            
Customer Requested Rate on Equity 9.15%aee_CustomerRequestedRateonEquity
/ us-gaap_LitigationCaseAxis
= aee_PendingFercCaseMember
/ us-gaap_RegulatoryAgencyAxis
= aee_MidwestIndependentTransmissionSystemOperatorIncMember		          
Rate of return on common equity 12.38%aee_RateOfReturnOnCommonEquity
/ us-gaap_LitigationCaseAxis
= aee_PendingFercCaseMember
/ us-gaap_RegulatoryAgencyAxis
= aee_MidwestIndependentTransmissionSystemOperatorIncMember		          
Incentive adder to FERC allowed base return on common equity       50aee_IncentiveaddertoFERCallowedbasereturnoncommonequity
/ us-gaap_LitigationCaseAxis
= aee_PendingFercCaseMember
/ us-gaap_RegulatoryAgencyAxis
= aee_MidwestIndependentTransmissionSystemOperatorIncMember		    
New England Transmission Owners [Member] | Pending Ferc Case [Member]            
Rate And Regulatory Matters [Line Items]            
Public Utilities, Approved Return on Equity, Percentage 11.14%us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage
/ us-gaap_LitigationCaseAxis
= aee_PendingFercCaseMember
/ us-gaap_RegulatoryAgencyAxis
= aee_NewEnglandTransmissionOwnersMember		          
Wholesale Distribution Rate Case            
Rate And Regulatory Matters [Line Items]            
Current regulatory liabilities           24us-gaap_RegulatoryLiabilityCurrent
/ us-gaap_RegulatoryAssetAxis
= aee_WholesaleDistributionRateCaseMember
Subsequent Event | Midwest Independent Transmission System Operator, Inc [Member] | Pending Ferc Case [Member]            
Rate And Regulatory Matters [Line Items]            
Customer Requested Rate on Equity         8.67%aee_CustomerRequestedRateonEquity
/ us-gaap_LitigationCaseAxis
= aee_PendingFercCaseMember
/ us-gaap_RegulatoryAgencyAxis
= aee_MidwestIndependentTransmissionSystemOperatorIncMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		  
Minimum | New England Transmission Owners [Member] | Pending Ferc Case [Member]            
Rate And Regulatory Matters [Line Items]            
Public Utilities, Approved Return on Equity, Percentage 10.57%us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage
/ us-gaap_LitigationCaseAxis
= aee_PendingFercCaseMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_RegulatoryAgencyAxis
= aee_NewEnglandTransmissionOwnersMember		          
Maximum | New England Transmission Owners [Member] | Pending Ferc Case [Member]            
Rate And Regulatory Matters [Line Items]            
Public Utilities, Approved Return on Equity, Percentage 11.74%us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage
/ us-gaap_LitigationCaseAxis
= aee_PendingFercCaseMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_RegulatoryAgencyAxis
= aee_NewEnglandTransmissionOwnersMember		          
Ameren Illinois Company            
Rate And Regulatory Matters [Line Items]            
Current regulatory liabilities 84us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 159us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		        
Purchased power 343us-gaap_CostOfPurchasedPower
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 380us-gaap_CostOfPurchasedPower
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 705us-gaap_CostOfPurchasedPower
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		      
Miscellaneous income 17us-gaap_OtherNonoperatingIncome
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 10us-gaap_OtherNonoperatingIncome
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 7us-gaap_OtherNonoperatingIncome
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		      
Union Electric Company            
Rate And Regulatory Matters [Line Items]            
Current regulatory liabilities 18us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 57us-gaap_RegulatoryLiabilityCurrent
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		        
Purchased power 119us-gaap_CostOfPurchasedPower
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 127us-gaap_CostOfPurchasedPower
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 78us-gaap_CostOfPurchasedPower
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		      
Miscellaneous income 60us-gaap_OtherNonoperatingIncome
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 58us-gaap_OtherNonoperatingIncome
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 63us-gaap_OtherNonoperatingIncome
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		      
Union Electric Company | Entergy Refund            
Rate And Regulatory Matters [Line Items]            
Proceeds from Legal Settlements   31us-gaap_ProceedsFromLegalSettlements
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ us-gaap_RegulatoryAssetAxis
= aee_EntergyRefundMember		        
Purchased power     24us-gaap_CostOfPurchasedPower
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ us-gaap_RegulatoryAssetAxis
= aee_EntergyRefundMember		      
Miscellaneous income     5us-gaap_OtherNonoperatingIncome
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ us-gaap_RegulatoryAssetAxis
= aee_EntergyRefundMember		      
Reduction To Under-recovered Asset     2aee_ReductionToUnderRecoveredAsset
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ us-gaap_RegulatoryAssetAxis
= aee_EntergyRefundMember		      
New Nuclear Energy Center COL [Member] | Union Electric Company            
Rate And Regulatory Matters [Line Items]            
Investments in Power and Distribution Projects 69us-gaap_InvestmentsInPowerAndDistributionProjects
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ us-gaap_PublicUtilityAxis
= aee_NewNuclearEnergyCenterColMember		          
New Nuclear Energy Center COL [Member] | Union Electric Company | Maximum            
Rate And Regulatory Matters [Line Items]            
Investments in Power and Distribution Projects 100us-gaap_InvestmentsInPowerAndDistributionProjects
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ us-gaap_PublicUtilityAxis
= aee_NewNuclearEnergyCenterColMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		          
Pending Ferc Case [Member] | Ameren Illinois Company | Maximum            
Rate And Regulatory Matters [Line Items]            
Loss Contingency, Estimate of Possible Loss 22us-gaap_LossContingencyEstimateOfPossibleLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
/ us-gaap_PublicUtilityAxis
= aee_PendingFercCaseMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		          
Pending Ferc Case [Member] | Union Electric Company | Power Purchase Agreement With Entergy Arkansas            
Rate And Regulatory Matters [Line Items]            
Purchased power $ 8us-gaap_CostOfPurchasedPower
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ aee_RateOrderAxis
= aee_PendingFercCaseMember
/ us-gaap_RegulatoryAssetAxis
= aee_PowerPurchaseAgreementWithEntergyArkansasMember		          
[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.