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Commitments And Contingencies (Asbestos-Related Litigation And Tax Exemptions And Credits) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Asbestos-Related [Member]
 
Average number of total defendants named 80
Number of pending asbestos lawsuits as of the balance sheet date 93 [1]
Asbestos liabilities $ 18
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers 90.00%
Asbestos trust fund balance 23
Percent of difference to be contributed to the asbestos trust fund if cash expenditures are less than amount included in base electric rates. 90.00%
Claimed Manufacturing Exemptions And Credits [Member]
 
Loss contingency range of possible loss maximum $ 27
Minimum [Member] | Asbestos-Related [Member]
 
Average number of total defendants named two
Maximum [Member] | Asbestos-Related [Member]
 
Average number of total defendants named 272
[1] Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.