Commitments And Contingencies (Asbestos-Related Litigation) (Detail) - Jun. 30, 2015 - Asbestos-Related $ in Millions		USD ($) defendant lawsuit
Loss Contingencies [Line Items]
Average number of total defendants named | defendant		73
Asbestos-related lawsuits were pending	[1]	49
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		28
Range of possible loss, minimum | $		$ 3
Ameren Illinois Company
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending		36
Range of possible loss, minimum | $		$ 5
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.