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Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Income Taxes [Abstract]      
Statutory federal income tax rate 35.00% 35.00% 35.00%
Non-deductible impairment of goodwill   32.00%  
Depreciation differences (1.00%) (4.00%) (1.00%)
Amortization of investment tax credit (1.00%) (2.00%) (1.00%)
State tax rate 4.00% 8.00% 5.00%
Reserve for uncertain tax positions   (1.00%) (1.00%)
Permanent items     (1.00%) [1]
Tax credits   (3.00%) (1.00%)
Change in federal tax law   3.00% [2]  
Effective income tax rate 37.00% 68.00% 35.00%
[1] Permanent items are treated differently for book and tax purposes and primarily include Internal Revenue Code Section 199 production activity deductions for Ameren and Genco, company-owned life insurance for Ameren and Ameren Illinois, employee stock ownership plan dividends for Ameren, and nondeductible expenses for Ameren Illinois.
[2] 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 Bill of 2010.