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Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Taxes [Line Items]      
Federal statutory corporate income tax rate 21.00% 21.00% 21.00%
Amortization of excess deferred income taxes (9.00%) (7.00%) (4.00%)
Amortization of deferred investment tax credit (1.00%) (1.00%) (1.00%)
State tax 5.00% 6.00% 6.00%
Stock-based compensation (1.00%) (1.00%)  
Other depreciation differences     0.00%
TCJA     1.00%
Tax Credits     0.00%
Other permanent items     (1.00%)
Effective income tax rate 15.00% 18.00% 22.00%
Union Electric Company      
Income Taxes [Line Items]      
Federal statutory corporate income tax rate 21.00% 21.00% 21.00%
Amortization of excess deferred income taxes (16.00%) (11.00%) (4.00%)
Amortization of deferred investment tax credit (1.00%) (1.00%) (1.00%)
State tax 3.00% 5.00% 4.00%
Stock-based compensation 0.00% 0.00%  
Other depreciation differences     0.00%
TCJA     1.00%
Tax Credits     (1.00%)
Other permanent items     0.00%
Effective income tax rate 7.00% 14.00% 20.00%
Ameren Illinois Company      
Income Taxes [Line Items]      
Federal statutory corporate income tax rate 21.00% 21.00% 21.00%
Amortization of excess deferred income taxes (3.00%) (4.00%) (4.00%)
Amortization of deferred investment tax credit (1.00%) 0.00% 0.00%
State tax 7.00% 7.00% 7.00%
Stock-based compensation 0.00% 0.00%  
Other depreciation differences     (1.00%)
TCJA     1.00%
Tax Credits     0.00%
Other permanent items     0.00%
Effective income tax rate 24.00% 24.00% 24.00%