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Derivative Financial Instruments Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Offsetting Assets and Liabilities [Line Items]    
Gross Amounts Recognized in the Balance Sheet $ 19us-gaap_DerivativeFairValueOfDerivativeAsset [1] $ 34us-gaap_DerivativeFairValueOfDerivativeAsset [1]
Derivative Instruments 5us-gaap_DerivativeAssetFairValueGrossLiability 10us-gaap_DerivativeAssetFairValueGrossLiability
Net Amount 14us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral 24us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral
Gross Amounts Recognized in the Balance Sheet 232us-gaap_DerivativeFairValueOfDerivativeLiability [1] 178us-gaap_DerivativeFairValueOfDerivativeLiability [1]
Derivative Instruments 5us-gaap_DerivativeLiabilityFairValueGrossAsset 10us-gaap_DerivativeLiabilityFairValueGrossAsset
Cash Collateral Received/Posted 5us-gaap_DerivativeCollateralRightToReclaimCash [2] 24us-gaap_DerivativeCollateralRightToReclaimCash [2]
Net Amount 222us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral 144us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral
Union Electric Company
   
Offsetting Assets and Liabilities [Line Items]    
Gross Amounts Recognized in the Balance Sheet 18us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
[1] 33us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
[1]
Derivative Instruments 5us-gaap_DerivativeAssetFairValueGrossLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
9us-gaap_DerivativeAssetFairValueGrossLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Net Amount 13us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
24us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Gross Amounts Recognized in the Balance Sheet 46us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
[1] 24us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
[1]
Derivative Instruments 5us-gaap_DerivativeLiabilityFairValueGrossAsset
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
9us-gaap_DerivativeLiabilityFairValueGrossAsset
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Cash Collateral Received/Posted 5us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
[2] 9us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
[2]
Net Amount 36us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
6us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company
   
Offsetting Assets and Liabilities [Line Items]    
Gross Amounts Recognized in the Balance Sheet 1us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
1us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Derivative Instruments   1us-gaap_DerivativeAssetFairValueGrossLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Net Amount 1us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
 
Gross Amounts Recognized in the Balance Sheet 186us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
[1] 154us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
[1]
Derivative Instruments   1us-gaap_DerivativeLiabilityFairValueGrossAsset
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Cash Collateral Received/Posted   15us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
[2]
Net Amount $ 186us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 138us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
[1] The derivative asset and liability balances are presented net of counterparty credit considerations.
[2] Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.