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Commitments And Contingencies (Other Obligations) (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
MWh
Long-term Purchase Commitment [Line Items]  
Total Other Obligations $ 5,326us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
Union Electric Company  
Long-term Purchase Commitment [Line Items]  
Total Other Obligations 3,577us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company  
Long-term Purchase Commitment [Line Items]  
Total Other Obligations 1,702us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Amount of Megawatthours 5,526,000aee_AmountofMegawatthours
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Long-term Purchase Commitment, Amount $ 185us-gaap_LongTermPurchaseCommitmentAmount
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember