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Commitments and Contingencies (Asbestos-Related Litigation) (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2011
Asbestos-Related [Member] | Minimum [Member]
 
Average number of total defendants named two
Asbestos-Related [Member] | Maximum [Member]
 
Average number of total defendants named 212
Asbestos-Related [Member]
 
Average number of total defendants named 78
Number of pending asbestos lawsuits as of the balance sheet date 95 [1]
Asbestos liabilities 19
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 19
[1] Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.