Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			0 Months Ended	9 Months Ended	3 Months Ended	1 Months Ended
	Jun. 30, 2011	Sep. 30, 2011	Dec. 31, 2010	Oct. 21, 2011 Voluntary Separation Offer [Member] Ameren Missouri [Member]	Sep. 30, 2011 Ameren Energy Generating Company [Member] Shutdown Of Meredosia And Hutsonville Energy Centers [Member]	Jun. 30, 2011 Ameren Missouri [Member]	Jun. 30, 2011 Ameren Energy Generating Company [Member]	Sep. 30, 2011 Ameren Energy Generating Company [Member]	Dec. 31, 2010 Ameren Energy Generating Company [Member]	Sep. 30, 2011 Renewable Energy Credits [Member]
Summary Of Significant Accounting Policies [Line Items]
Pretax impairment charge	$ 2					$ 1
Book value		6	7						3	5
Unrecognized tax benefits		189
Reduction of uncertain tax liabilities	39
Unrecognized tax benefits that would impact effective tax rate		3
Percentage of EEI not owned by Ameren		20.00%
Proceeds from sale of machinery and equipment							45
Pretax gain recognized on sale of machinery and equipment							8
Number of employee positions eliminated				715	90
Non-cash impairment of plant book value					26
Non-cash impairment of materials and supplies					5
Severance costs					4
Expected tax benefits related to closure of plants					22
AROs associated with energy center closures		439	475		38			69	74
Expected tax benefits related to asset retirement obligations					$ 16