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Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Line of Credit Facility [Line Items]    
Commercial paper outstanding $ 955us-gaap_ShortTermBorrowings $ 714us-gaap_ShortTermBorrowings
Ameren (parent)    
Line of Credit Facility [Line Items]    
Commercial paper outstanding 815us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= aee_AmerenparentDomain		 585us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= aee_AmerenparentDomain
Union Electric Company    
Line of Credit Facility [Line Items]    
Commercial paper outstanding 140us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember		 97us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company    
Line of Credit Facility [Line Items]    
Commercial paper outstanding $ 0us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember		 $ 32us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember