Commitments And Contingencies (Asbestos-Related Litigation And Tax Exemptions And Credits) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 defendant lawsuit
Asbestos-Related [Member]
Loss Contingencies [Line Items]
Average number of total defendants named	80
Asbestos-related lawsuits were pending	103	[1]
Asbestos liabilities	$ 19.0
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%
Asbestos-Related [Member] | Union Electric Company [Member]
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending	66
Asbestos liabilities	8.0
Asbestos-Related [Member] | Ameren Illinois Company [Member]
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending	81
Asbestos liabilities	11.0
Asbestos-Related [Member] | Ameren Corporation [Member]
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending	3
Collectibility of Taxes [Member] | Ameren Illinois Company [Member]
Loss Contingencies [Line Items]
Asbestos liabilities	2.0
Public Utilities, Number of Customers	2,100
Loss contingency range of possible loss maximum	4.0
Claimed Manufacturing Exemptions And Credits [Member]
Loss Contingencies [Line Items]
Loss contingency range of possible loss maximum	$ 27.0

[1] Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.