Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments)) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	$ 0	$ 54
2013 Tax Year
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	53
Union Electric Company
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	0	0
Ameren Illinois Company
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	$ 0	$ (1)