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Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)
3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Income Taxes [Line Items]    
Federal statutory corporate income tax rate 21.00% 21.00%
Increases (decreases) from:    
Amortization of deferred investment tax credit 0.00% (1.00%)
Amortization of excess deferred taxes (8.00%) (9.00%)
Depreciation differences 0.00% 0.00%
Renewable and other tax credits (5.00%) (6.00%)
State tax 5.00% 6.00%
Stock-based compensation (1.00%) (1.00%)
Effective income tax rate 12.00% 10.00%
Union Electric Company    
Income Taxes [Line Items]    
Federal statutory corporate income tax rate 21.00% 21.00%
Increases (decreases) from:    
Amortization of deferred investment tax credit (1.00%) (1.00%)
Amortization of excess deferred taxes (16.00%) (17.00%)
Depreciation differences 0.00% 1.00%
Renewable and other tax credits (11.00%) (11.00%)
State tax 3.00% 3.00%
Stock-based compensation 0.00% 0.00%
Effective income tax rate (4.00%) (4.00%)
Ameren Illinois Company    
Income Taxes [Line Items]    
Federal statutory corporate income tax rate 21.00% 21.00%
Increases (decreases) from:    
Amortization of deferred investment tax credit 0.00% 0.00%
Amortization of excess deferred taxes (2.00%) (2.00%)
Depreciation differences 0.00% (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 26.00% 25.00%