Summary of Significant Accounting Policies (Schedule of Noncontrolling Interest) (Details) (USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Summary of Significant Accounting Policies
Noncontrolling interest, beginning of period	$ 155		$ 206		$ 154		$ 204
Net income attributable to noncontrolling interest	1	[1]	3	[1]	4	[1]	7	[1]
Dividends paid to noncontrolling interest holders	(1)		(3)		(3)		(5)
Noncontrolling interest, end of period	$ 155		$ 206		$ 155		$ 206

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