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Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Derivative [Line Items]  
Maximum exposure to counterparties related to derivative contracts $ 6us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
Union Electric Company  
Derivative [Line Items]  
Maximum exposure to counterparties related to derivative contracts 5us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company  
Derivative [Line Items]  
Maximum exposure to counterparties related to derivative contracts $ 1us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember