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Fair Value Measurements (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Valuation adjustments related to net derivative contracts, liabilities $ 1us-gaap_DerivativeCreditRiskValuationAdjustmentDerivativeLiabilities $ 1us-gaap_DerivativeCreditRiskValuationAdjustmentDerivativeLiabilities
Union Electric Company    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Valuation adjustments related to net derivative contracts, liabilities 1us-gaap_DerivativeCreditRiskValuationAdjustmentDerivativeLiabilities
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 1us-gaap_DerivativeCreditRiskValuationAdjustmentDerivativeLiabilities
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Valuation adjustments related to net derivative contracts, liabilities $ 1us-gaap_DerivativeCreditRiskValuationAdjustmentDerivativeLiabilities
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 $ 1us-gaap_DerivativeCreditRiskValuationAdjustmentDerivativeLiabilities
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember