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Income Taxes (Schedule Of Changes To Unrecognized Tax Benefits And Related Interest) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized tax benefits $ 90us-gaap_UnrecognizedTaxBenefits $ 156us-gaap_UnrecognizedTaxBenefits $ 148us-gaap_UnrecognizedTaxBenefits
Increases based on tax positions 6us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 7us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 5us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Decreases based on tax positions (42)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions (143)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions (13)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
Increases based on tax positions related to current period 0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 69us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 17us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Changes related to settlements with taxing authorities (20)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities  
Decreases related to the lapse of statute of limitations   1us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations (1)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
Unrecognized tax benefits 54us-gaap_UnrecognizedTaxBenefits 90us-gaap_UnrecognizedTaxBenefits 156us-gaap_UnrecognizedTaxBenefits
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates 52us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 54us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 1us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Union Electric Company      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized tax benefits 31us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
136us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
124us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Increases based on tax positions 1us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
4us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Decreases based on tax positions (32)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
(122)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
(7)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Increases based on tax positions related to current period 0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
16us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
15us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Changes related to settlements with taxing authorities (18)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
 
Decreases related to the lapse of statute of limitations   1us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
 
Unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
31us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
136us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates 0us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
3us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
3us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized tax benefits (1)us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
13us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
11us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Increases based on tax positions 1us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
2us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Decreases based on tax positions (1)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
(16)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
(1)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Increases based on tax positions related to current period 0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
  3us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Changes related to settlements with taxing authorities   0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
 
Unrecognized tax benefits (1)us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
(1)us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
13us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates (1)us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
  (1)us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Other      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized tax benefits 60us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
7us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
13us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
Increases based on tax positions 4us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
5us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
1us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
Decreases based on tax positions (9)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
(5)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
(5)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
Increases based on tax positions related to current period 0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
53us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
[1] (1)us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
Changes related to settlements with taxing authorities   0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
 
Decreases related to the lapse of statute of limitations     (1)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
Unrecognized tax benefits 55us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
60us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
7us-gaap_UnrecognizedTaxBenefits
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates $ 53us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
[1] $ 51us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
[1] $ (1)us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ dei_LegalEntityAxis
= aee_OtherAffiliatedEntitiesAndIntercompanyEliminationsMember
[1] Primarily due to tax positions relating to the New AER divestiture. The income statement impact of this unrecognized tax benefit was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information.