Commitments And Contingencies (Asbestos-Related Litigation) (Detail) - Asbestos-Related $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) defendant lawsuit
Loss Contingencies [Line Items]
Average number of total defendants named | defendant	73
Asbestos-related lawsuits were pending | lawsuit	53	[1]
Range of possible loss, minimum	$ 8
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers	90.00%
Asbestos trust fund balance	$ 22
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%
Union Electric Company
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending | lawsuit	27
Range of possible loss, minimum	$ 3
Ameren Illinois Company
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending | lawsuit	40
Range of possible loss, minimum	$ 5
Ameren Corporation
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending | lawsuit	1

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