Summary Of Significant Accounting Policies (Equity Changes Attributable To Noncontrolling Interest) (Details) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Noncontrolling interest, beginning of period	$ 155		$ 206		$ 154		$ 204
Net income attributable to noncontrolling interest	2	[1]	3	[1]	6	[1]	10	[1]
Dividends paid to noncontrolling interest holders	(2)		(2)		(5)		(7)
Purchase of subsidiary preferred shares from noncontrolling interests			(52)	[2]			(52)	[2]
Noncontrolling interest, end of period	155		155		155		155
CILCO [Member]
Preferred stock redemptions	19				19
Ameren Missouri [Member]
Preferred stock redemptions	$ 33				$ 33

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

[2] Represents preferred stock redemptions of $33 million and $19 million by Ameren Missouri and CILCO, respectively.