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Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Rate
Jun. 30, 2018
Rate
Jun. 30, 2019
Rate
Jun. 30, 2018
Rate
Income Taxes [Line Items]        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00% 21.00% 21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (9.00%) (1.00%) (8.00%) (2.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 0.00% 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent 0.00% 0.00% 0.00% (1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 6.00% 5.00% 6.00% 6.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent     (3.00%) (1.00%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent 0.00% (1.00%)    
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent     (1.00%) 0.00%
Effective Income Tax Rate Reconciliation, Percent 18.00% 24.00% 15.00% 23.00%
Union Electric Company        
Income Taxes [Line Items]        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00% 21.00% 21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (12.00%) 0.00% (12.00%) 0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 0.00% 1.00% 0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent (1.00%) (1.00%) (1.00%) (1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 4.00% 4.00% 4.00% 4.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent     0.00% 0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent 0.00% 0.00%    
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent     (1.00%) 0.00%
Effective Income Tax Rate Reconciliation, Percent 12.00% 24.00% 12.00% 24.00%
Ameren Illinois Company        
Income Taxes [Line Items]        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00% 21.00% 21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (4.00%) (5.00%) (4.00%) (4.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent (1.00%) (1.00%) 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent 0.00% 0.00% 0.00% 0.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 7.00% 8.00% 7.00% 7.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent     0.00% 0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent 0.00% 0.00%    
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent     0.00% 0.00%
Effective Income Tax Rate Reconciliation, Percent 23.00% 23.00% 24.00% 24.00%