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Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Taxes [Line Items]      
Federal statutory corporate income tax rate: 21.00% 35.00% 35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (4.00%)    
Other depreciation differences 0.00%    
Amortization of deferred investment tax credit (1.00%) (1.00%) (0.00%)
State tax 6.00% 6.00% 4.00%
TCJA 1.00% 14.00%  
Tax Credits (0.00%) (0.00%)  
Stock-based compensation     (2.00%)
Other permanent items (1.00%) (2.00%) (1.00%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent     1.00%
Effective income tax rate 22.00% 52.00% 37.00%
Adjustments related to Income Tax Benefit from Share-Based Payment     $ 21
Union Electric Company      
Income Taxes [Line Items]      
Federal statutory corporate income tax rate: 21.00% 35.00% 35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (4.00%)    
Other depreciation differences 0.00% 1.00% 1.00%
Amortization of deferred investment tax credit (1.00%) (1.00%) (1.00%)
State tax 4.00% 4.00% 3.00%
TCJA 1.00% 6.00%  
Tax Credits (1.00%) (1.00%)  
Stock-based compensation     0.00%
Other permanent items 0.00% 0.00% 0.00%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent     0.00%
Effective income tax rate 20.00% 44.00% 38.00%
Ameren Illinois Company      
Income Taxes [Line Items]      
Federal statutory corporate income tax rate: 21.00% 35.00% 35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent (4.00%)    
Other depreciation differences (1.00%) (1.00%)  
Amortization of deferred investment tax credit (0.00%) (0.00%) (0.00%)
State tax 7.00% 6.00% 5.00%
TCJA 1.00% (1.00%)  
Tax Credits (0.00%) (0.00%)  
Stock-based compensation     0.00%
Other permanent items   (1.00%) (2.00%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent     0.00%
Effective income tax rate 24.00% 38.00% 38.00%