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Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Income Taxes [Line Items]    
Federal statutory corporate income tax rate 21.00% 21.00%
Amortization of deferred investment tax credit (1.00%) 0.00%
Amortization of excess deferred taxes (9.00%) (9.00%)
Depreciation differences 0.00% (1.00%)
Renewable and other tax credits (6.00%) 0.00%
State tax 6.00% 6.00%
Stock-based compensation (1.00%) (5.00%)
Effective income tax rate 10.00% 12.00%
Union Electric Company    
Income Taxes [Line Items]    
Federal statutory corporate income tax rate 21.00% 21.00%
Amortization of deferred investment tax credit (1.00%) (1.00%)
Amortization of excess deferred taxes (17.00%) (15.00%)
Depreciation differences 1.00% 0.00%
Renewable and other tax credits (11.00%) 0.00%
State tax 3.00% 3.00%
Stock-based compensation 0.00% 0.00%
Effective income tax rate (4.00%) 8.00%
Ameren Illinois Company    
Income Taxes [Line Items]    
Federal statutory corporate income tax rate 21.00% 21.00%
Amortization of deferred investment tax credit 0.00% 0.00%
Amortization of excess deferred taxes (2.00%) (3.00%)
Depreciation differences (1.00%) (1.00%)
Renewable and other tax credits 0.00% 0.00%
State tax 7.00% 7.00%
Stock-based compensation 0.00% 0.00%
Effective income tax rate 25.00% 24.00%