Commitments And Contingencies (Asbestos-Related Litigation) (Detail) (Asbestos-Related, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 defendant lawsuit
Loss Contingencies [Line Items]
Average number of total defendants named	77
Asbestos-related lawsuits were pending	65	[1]
Range of possible loss, minimum	$ 12.0
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers	90.00%
Asbestos trust fund balance	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	42
Range of possible loss, minimum	5.0
Ameren Illinois Company
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending	53
Range of possible loss, minimum	$ 7.0
Ameren Corporation
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending	1

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