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Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Income Taxes [Line Items]      
Statutory federal income tax rate: 35.00% 35.00% 35.00%
Non-deductible impairment of goodwill     32.00%
Depreciation differences    (1.00%) (4.00%)
Amortization of investment tax credit 1.00% (1.00%) (2.00%)
State tax 5.00% 4.00% 8.00%
Reserve for uncertain tax positions      (1.00%)
Other permanent items       
Tax credits     (3.00%)
Change in federal tax law     3.00% [1]
Effective income tax rate 41.00% 37.00% 68.00%
Union Electric Company [Member]
     
Income Taxes [Line Items]      
Statutory federal income tax rate: 35.00% 35.00% 35.00%
Non-deductible impairment of goodwill       
Depreciation differences (1.00%) (2.00%) (3.00%)
Amortization of investment tax credit (1.00%) (1.00%) (1.00%)
State tax 3.00% 3.00% 3.00%
Reserve for uncertain tax positions 1.00%     
Other permanent items   1.00% [2]  
Tax credits       
Change in federal tax law     1.00% [1]
Effective income tax rate 37.00% 36.00% 35.00%
Ameren Illinois Company [Member]
     
Income Taxes [Line Items]      
Statutory federal income tax rate: 35.00% 35.00% 35.00%
Non-deductible impairment of goodwill       
Depreciation differences         
Amortization of investment tax credit (1.00%) (1.00%) (1.00%)
State tax 6.00% 5.00% 5.00%
Reserve for uncertain tax positions        
Other permanent items       
Tax credits       
Change in federal tax law        [1]
Effective income tax rate 40.00% 39.00% 39.00%
[1] Relates to change in taxation of prescription drug benefits to retiree participants from the enactment in 2010 of the Patient Protection and Affordable Care Act and the Health Care and Education Reconciliation Act of 2010.
[2] Permanent items are treated differently for book and tax purposes and primarily include nondeductible expenses related to lobbying and stock issuance expenses for Ameren Missouri.