Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact the Effective Tax Rate) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]
Unrecognized tax benefits (detriments) that would impact effective tax rate		$ 52
Union Electric Company
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]
Unrecognized tax benefits (detriments) that would impact effective tax rate		0
Ameren Illinois Company
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]
Unrecognized tax benefits (detriments) that would impact effective tax rate		$ (1)