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Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Selected Quarterly Financial Information [Line Items]                      
Operating revenues $ 1,370us-gaap_Revenues $ 1,670us-gaap_Revenues $ 1,419us-gaap_Revenues $ 1,594us-gaap_Revenues $ 1,322us-gaap_Revenues $ 1,638us-gaap_Revenues $ 1,403us-gaap_Revenues $ 1,475us-gaap_Revenues $ 6,053us-gaap_Revenues $ 5,838us-gaap_Revenues $ 5,781us-gaap_Revenues
Operating Income 125us-gaap_OperatingIncomeLoss 561us-gaap_OperatingIncomeLoss 322us-gaap_OperatingIncomeLoss 246us-gaap_OperatingIncomeLoss 171us-gaap_OperatingIncomeLoss 567us-gaap_OperatingIncomeLoss 261us-gaap_OperatingIncomeLoss 185us-gaap_OperatingIncomeLoss 1,254us-gaap_OperatingIncomeLoss 1,184us-gaap_OperatingIncomeLoss 1,188us-gaap_OperatingIncomeLoss
Net income (loss) 49us-gaap_ProfitLoss 295us-gaap_ProfitLoss 150us-gaap_ProfitLoss 98us-gaap_ProfitLoss 38us-gaap_ProfitLoss 304us-gaap_ProfitLoss 96us-gaap_ProfitLoss (143)us-gaap_ProfitLoss 592us-gaap_ProfitLoss 295us-gaap_ProfitLoss (974)us-gaap_ProfitLoss
Net income attributable to Ameren Corporation - continuing operations 46us-gaap_IncomeLossFromContinuingOperations 294us-gaap_IncomeLossFromContinuingOperations 150us-gaap_IncomeLossFromContinuingOperations 97us-gaap_IncomeLossFromContinuingOperations 48us-gaap_IncomeLossFromContinuingOperations 305us-gaap_IncomeLossFromContinuingOperations 105us-gaap_IncomeLossFromContinuingOperations 54us-gaap_IncomeLossFromContinuingOperations 587us-gaap_IncomeLossFromContinuingOperations 512us-gaap_IncomeLossFromContinuingOperations 516us-gaap_IncomeLossFromContinuingOperations
Net Income (Loss) Attributable to Ameren Corporation - Discontinued Operations 2us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (1)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (1)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (1)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (11)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (3)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (10)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (199)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (1)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (223)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity (1,490)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity
Net income (loss) attributable to Ameren Corporation 48us-gaap_NetIncomeLoss 293us-gaap_NetIncomeLoss 149us-gaap_NetIncomeLoss 96us-gaap_NetIncomeLoss 37us-gaap_NetIncomeLoss 302us-gaap_NetIncomeLoss 95us-gaap_NetIncomeLoss (145)us-gaap_NetIncomeLoss 586us-gaap_NetIncomeLoss 289us-gaap_NetIncomeLoss (974)us-gaap_NetIncomeLoss
Earnings per common share - basic - continuing operations $ 0.19us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 1.21us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.62us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.40us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.19us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 1.26us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.44us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.22us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 2.42us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 2.11us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 2.13us-gaap_IncomeLossFromContinuingOperationsPerBasicShare
Earnings (loss) per common share - basic - discontinued operations $ 0.01us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare   $ (0.01)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ (0.04)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ (0.01)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ (0.05)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ (0.82)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ (0.92)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ (6.14)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare
Earnings (loss) per common share - basic $ 0.20us-gaap_EarningsPerShareBasic $ 1.21us-gaap_EarningsPerShareBasic $ 0.61us-gaap_EarningsPerShareBasic $ 0.40us-gaap_EarningsPerShareBasic $ 0.15us-gaap_EarningsPerShareBasic $ 1.25us-gaap_EarningsPerShareBasic $ 0.39us-gaap_EarningsPerShareBasic $ (0.60)us-gaap_EarningsPerShareBasic $ 2.42us-gaap_EarningsPerShareBasic $ 1.19us-gaap_EarningsPerShareBasic $ (4.01)us-gaap_EarningsPerShareBasic
Earnings per common share - diluted - continuing operations $ 0.19us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 1.20us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.62us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.40us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.19us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 1.25us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.44us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.22us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 2.40us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 2.10us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 2.13us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare
Earnings (loss) per common share - diluted - discontinued operations $ 0.01us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ (0.01)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ (0.04)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ (0.01)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ (0.05)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ (0.82)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ (0.92)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ (6.14)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare
Earnings (loss) per common share - diluted $ 0.20us-gaap_EarningsPerShareDiluted $ 1.20us-gaap_EarningsPerShareDiluted $ 0.61us-gaap_EarningsPerShareDiluted $ 0.40us-gaap_EarningsPerShareDiluted $ 0.15us-gaap_EarningsPerShareDiluted $ 1.24us-gaap_EarningsPerShareDiluted $ 0.39us-gaap_EarningsPerShareDiluted $ (0.60)us-gaap_EarningsPerShareDiluted $ 2.40us-gaap_EarningsPerShareDiluted $ 1.18us-gaap_EarningsPerShareDiluted $ (4.01)us-gaap_EarningsPerShareDiluted
Union Electric Company                      
Selected Quarterly Financial Information [Line Items]                      
Operating revenues 739us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
1,097us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
900us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
817us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
763us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
1,093us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
889us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
796us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
3,553us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
3,541us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
3,272us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Operating Income 29us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
394us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
243us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
119us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
96us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
417us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
179us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
111us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
785us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
803us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
845us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Net income (loss)                 393us-gaap_ProfitLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
398us-gaap_ProfitLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
419us-gaap_ProfitLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Net income (loss) attributable to Ameren Corporation (5)us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
223us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
127us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
48us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
33us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
239us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
85us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
41us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
393us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
398us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
419us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Net Income Available to Common Stockholder (5)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
222us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
126us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
47us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
33us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
238us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
84us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
40us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
390us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
395us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
416us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_UnionElectricCompanyMember
Ameren Illinois Company                      
Selected Quarterly Financial Information [Line Items]                      
Operating revenues 633us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
572us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
519us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
774us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
564us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
547us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
516us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
684us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
2,498us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
2,311us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
2,525us-gaap_Revenues
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Operating Income 97us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
158us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
75us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
120us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
85us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
158us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
87us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
85us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
450us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
415us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
377us-gaap_OperatingIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Net income (loss)                 204us-gaap_ProfitLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
163us-gaap_ProfitLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
144us-gaap_ProfitLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Net income (loss) attributable to Ameren Corporation 46us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
75us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
29us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
54us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
22us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
77us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
32us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
32us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
204us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
163us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
144us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
Net Income Available to Common Stockholder $ 45us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 75us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 28us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 53us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 21us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 77us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 31us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 31us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 201us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 160us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember
$ 141us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ dei_LegalEntityAxis
= aee_AmerenIllinoisCompanyMember