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Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
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 (7.00%) (6.00%) (7.00%) (3.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 0.00%    
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent (1.00%) (1.00%) (1.00%) (1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 6.00% 6.00% 6.00% 6.00%
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent 1.00% 0.00% (1.00%) 0.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 0.00% 3.00% 0.00% 1.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent     0.00% (1.00%)
Effective Income Tax Rate Reconciliation, Percent 20.00% 23.00% 18.00% 23.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability       $ 13
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 (11.00%) (7.00%) (12.00%) (4.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% 0.00%    
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent (1.00%) 0.00% (1.00%) 0.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 5.00% 4.00% 5.00% 4.00%
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent 0.00% 0.00% 0.00% 0.00%
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%
Effective Income Tax Rate Reconciliation, Percent 14.00% 18.00% 13.00% 21.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability       $ 4
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 (3.00%) (3.00%) (4.00%) (4.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent 0.00% (1.00%)    
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 6.00% 5.00% 7.00% 7.00%
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent 0.00% 0.00% 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 0.00% 4.00% 0.00% 1.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent     0.00% 0.00%
Effective Income Tax Rate Reconciliation, Percent 24.00% 26.00% 24.00% 25.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability       $ 4