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, unless otherwise specified	Mar. 31, 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	$ 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	(1)	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)	$ (1)