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Commitments And Contingencies (Asbestos-Related Litigation) (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2011
Minimum [Member] | Asbestos-Related [Member]
 
Average number of total defendants namedtwo
Maximum [Member] | Asbestos-Related [Member]
 
Average number of total defendants named221
Asbestos-Related [Member]
 
Average number of total defendants named79
Number of pending asbestos lawsuits as of the balance sheet date107[1]
Asbestos liabilities 20
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers90.00%
Asbestos trust fund balance23
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 23
[1]Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.