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Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Taxes [Line Items]      
Statutory federal income tax rate: 35.00% 35.00% 35.00%
Depreciation differences 0.00% (1.00%)  
Amortization of deferred investment tax credit (0.00%) (1.00%) (1.00%)
State tax 4.00% 5.00% 4.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent [1] (2.00%)    
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent 1.00%    
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent (1.00%)   1.00%
Effective income tax rate 37.00% 38.00% 39.00%
Union Electric Company      
Income Taxes [Line Items]      
Statutory federal income tax rate: 35.00% 35.00% 35.00%
Depreciation differences 1.00% 0.00%  
Amortization of deferred investment tax credit (1.00%) (1.00%) (1.00%)
State tax 3.00% 3.00% 3.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent [1] 0.00%    
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent 0.00%    
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent 0.00% 0.00% 0.00%
Effective income tax rate 38.00% 37.00% 37.00%
Ameren Illinois Company      
Income Taxes [Line Items]      
Statutory federal income tax rate: 35.00% 35.00% 35.00%
Depreciation differences 0.00% (2.00%)  
Amortization of deferred investment tax credit (0.00%) (0.00%) (0.00%)
State tax 5.00% 5.00% 6.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent [1] 0.00%    
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent 0.00%    
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent (2.00%) (1.00%)  
Effective income tax rate 38.00% 37.00% 41.00%
[1] (a)Reflects the adoption of new authoritative accounting guidance related to share-based compensation. See Note 1 – Summary of Significant Accounting Policies for more information.