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Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)
3 Months Ended
Mar. 31, 2019
Rate
Mar. 31, 2018
Rate
Income Taxes [Line Items]    
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (7.00%) (2.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent (1.00%) (1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 6.00% 7.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent (7.00%) (3.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Percent 12.00% 22.00%
Union Electric Company    
Income Taxes [Line Items]    
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (12.00%) 0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 1.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent (1.00%) (1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 4.00% 4.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent (3.00%) 0.00%
Effective Income Tax Rate Reconciliation, Percent 9.00% 25.00%
Ameren Illinois Company    
Income Taxes [Line Items]    
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (3.00%) (3.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent (0.00%) (0.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 7.00% 7.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Percent 25.00% 25.00%