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Long-Term Debt and Equity Financings (Schedule of Required and Actual Debt Ratios) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Union Electric Company  
Debt Instrument [Line Items]  
Bonds Issuable Based On Coverage Ratio 3,605aee_BondsIssuableBasedOnCoverageRatio
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
[1]
Preferred Stock Issuable Based On Coverage Ratio 2,568aee_PreferredStockIssuableBasedOnCoverageRatio
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Retired Bond Capacity 832aee_RetiredBondCapacity
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Union Electric Company | Actual Interest Coverage Ratio  
Debt Instrument [Line Items]  
Interest Coverage Ratio 4.3aee_InterestCoverageRatio
/ aee_DebtRatioAxis
= aee_ActualRatioMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Dividend Coverage Ratio 115.1aee_DividendCoverageRatio
/ aee_DebtRatioAxis
= aee_ActualRatioMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company  
Debt Instrument [Line Items]  
Bonds Issuable Based On Coverage Ratio 3,358aee_BondsIssuableBasedOnCoverageRatio
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
[1],[2]
Preferred Stock Issuable Based On Coverage Ratio 208aee_PreferredStockIssuableBasedOnCoverageRatio
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Retired Bond Capacity 204aee_RetiredBondCapacity
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Ameren Illinois Company | Actual Interest Coverage Ratio  
Debt Instrument [Line Items]  
Interest Coverage Ratio 6.4aee_InterestCoverageRatio
/ aee_DebtRatioAxis
= aee_ActualRatioMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Dividend Coverage Ratio 2.7aee_DividendCoverageRatio
/ aee_DebtRatioAxis
= aee_ActualRatioMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Minimum | Union Electric Company | Required Dividend Coverage Ratio  
Debt Instrument [Line Items]  
Interest Coverage Ratio 2.0aee_InterestCoverageRatio
/ aee_DebtRatioAxis
= aee_MinimumRequiredRatioMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
[3]
Dividend Coverage Ratio 2.5aee_DividendCoverageRatio
/ aee_DebtRatioAxis
= aee_MinimumRequiredRatioMember
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
[4]
Minimum | Ameren Illinois Company | Required Dividend Coverage Ratio  
Debt Instrument [Line Items]  
Interest Coverage Ratio 2.0aee_InterestCoverageRatio
/ aee_DebtRatioAxis
= aee_MinimumRequiredRatioMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
[3]
Dividend Coverage Ratio 1.5aee_DividendCoverageRatio
/ aee_DebtRatioAxis
= aee_MinimumRequiredRatioMember
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
[4]
[1] Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $832 million and $204 million at Ameren Missouri and Ameren Illinois, respectively.
[2] Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture.
[3] Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds.
[4] Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation.