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Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]      
Federal statutory corporate income tax rate: 35.00% 35.00% 35.00%
Depreciation differences 0.00%   (1.00%)
Amortization of deferred investment tax credit (1.00%) (0.00%) (1.00%)
State tax 6.00% 4.00% 5.00%
TCJA 14.00%    
Tax Credits (0.00%)    
Stock-based compensation   (2.00%)  
Valuation allowance   1.00%  
Other permanent items (2.00%) (1.00%)  
Effective income tax rate 52.00% 37.00% 38.00%
Adjustments related to Income Tax Benefit from Share-Based Payment   $ 21  
Union Electric Company      
Income Taxes [Line Items]      
Federal statutory corporate income tax rate: 35.00% 35.00% 35.00%
Depreciation differences 1.00% 1.00% 0.00%
Amortization of deferred investment tax credit (1.00%) (1.00%) (1.00%)
State tax 4.00% 3.00% 3.00%
TCJA 6.00%    
Tax Credits (1.00%)    
Stock-based compensation   0.00%  
Valuation allowance   0.00%  
Other permanent items 0.00% 0.00% 0.00%
Effective income tax rate 44.00% 38.00% 37.00%
Ameren Illinois Company      
Income Taxes [Line Items]      
Federal statutory corporate income tax rate: 35.00% 35.00% 35.00%
Depreciation differences (1.00%)   (2.00%)
Amortization of deferred investment tax credit (0.00%) (0.00%) (0.00%)
State tax 6.00% 5.00% 5.00%
TCJA (1.00%)    
Tax Credits (0.00%)    
Stock-based compensation   0.00%  
Valuation allowance   0.00%  
Other permanent items (1.00%) (2.00%) (1.00%)
Effective income tax rate 38.00% 38.00% 37.00%