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Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Rate
Sep. 30, 2017
Rate
Sep. 30, 2018
Rate
Sep. 30, 2017
Rate
Income Taxes [Line Items]        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 35.00% 21.00% 35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (6.00%) 0.00% (3.00%) 0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 1.00% 0.00% 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 6.00% 7.00% 6.00% 6.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 3.00% 0.00% 1.00% 0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent 0.00% (1.00%) (1.00%) (1.00%)
Effective Income Tax Rate Reconciliation, Percent 23.00% 41.00% 23.00% 39.00%
Union Electric Company        
Income Taxes [Line Items]        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 35.00% 21.00% 35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (7.00%) 0.00% (4.00%) 0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 1.00% 0.00% 1.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent (0.00%) (1.00%) (0.00%) (1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 4.00% 3.00% 4.00% 3.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 0.00% 0.00% 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent 0.00% 0.00% 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Percent 18.00% 38.00% 21.00% 38.00%
Ameren Illinois Company        
Income Taxes [Line Items]        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 35.00% 21.00% 35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (3.00%) 0.00% (4.00%) 0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent (1.00%) (2.00%) 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 5.00% 7.00% 7.00% 5.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 4.00% 0.00% 1.00% 0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent 0.00% 0.00% 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Percent 26.00% 40.00% 25.00% 40.00%