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Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Income Taxes [Line Items]        
Federal statutory corporate income tax rate 21.00% 21.00% 21.00% 21.00%
Amortization of deferred investment tax credit (1.00%) (1.00%) 0.00% (1.00%)
Amortization of excess deferred taxes (9.00%) (7.00%) (9.00%) (7.00%)
Depreciation differences 0.00% 0.00%    
Renewable and other tax credits (1.00%) 0.00% (1.00%) 0.00%
State tax 4.00% 6.00% 5.00% 6.00%
Stock-based compensation 1.00% 1.00% (1.00%) (1.00%)
Effective income tax rate 15.00% 20.00% 15.00% 18.00%
Union Electric Company        
Income Taxes [Line Items]        
Federal statutory corporate income tax rate 21.00% 21.00% 21.00% 21.00%
Amortization of deferred investment tax credit (1.00%) (1.00%) (1.00%) (1.00%)
Amortization of excess deferred taxes (16.00%) (11.00%) (16.00%) (12.00%)
Depreciation differences 0.00% 0.00%    
Renewable and other tax credits (2.00%) 0.00% (1.00%) 0.00%
State tax 4.00% 5.00% 3.00% 5.00%
Stock-based compensation 0.00% 0.00% 0.00% 0.00%
Effective income tax rate 6.00% 14.00% 6.00% 13.00%
Ameren Illinois Company        
Income Taxes [Line Items]        
Federal statutory corporate income tax rate 21.00% 21.00% 21.00% 21.00%
Amortization of deferred investment tax credit 0.00% 0.00% 0.00% 0.00%
Amortization of excess deferred taxes (3.00%) (3.00%) (3.00%) (4.00%)
Depreciation differences (1.00%) 0.00%    
Renewable and other tax credits 0.00% 0.00% 0.00% 0.00%
State tax 7.00% 6.00% 7.00% 7.00%
Stock-based compensation 0.00% 0.00% 0.00% 0.00%
Effective income tax rate 24.00% 24.00% 25.00% 24.00%