Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments)) (Details) (USD $) In Millions, unless otherwise specified							1 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Union Electric Company	Dec. 31, 2013 Union Electric Company	Sep. 30, 2014 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company	Oct. 31, 2014 Subsequent Event [Member]	Oct. 31, 2014 Subsequent Event [Member] Union Electric Company	Sep. 30, 2014 Tax Year 2013 [Member]	Sep. 30, 2014 Tax Year 2013 [Member] Union Electric Company	Sep. 30, 2014 Tax Year 2013 [Member] Ameren Illinois Company	Sep. 30, 2014 2011 and 2012 Tax Years [Member]	Sep. 30, 2014 New Ameren Energy Resources Company, LLC Tax Year 2013 [Member]
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefit recorded in Accumulated deferred income taxes, net	$ 89	$ 84	$ 13	$ 15	$ 2
Unrecognized tax benefits (detriments)	97	90	35	31	1	(1)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities							16	9
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	$ 18								$ 73	$ 17	$ 1	$ 6	$ 55