Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	12 Months Ended						12 Months Ended		1 Months Ended			12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009	Dec. 31, 2010 Emission Allowances [Member]	Dec. 31, 2011 Ameren Energy Generating Company [Member]	Dec. 31, 2010 Ameren Energy Generating Company [Member]	Feb. 29, 2012 Medina Valley Energy Center [Member]	Jun. 30, 2011 Columbia CT Energy Center [Member]	Jun. 30, 2010 Columbia CT Energy Center [Member]	Dec. 31, 2011 Shutdown Of Meredosia And Hutsonville Energy Centers [Member]	Dec. 31, 2010 Merchant Generation Separation Program [Member]	Dec. 31, 2009 Voluntary And Involuntary Separation Program [Member]	Dec. 31, 2011 Other Asset Sales [Member] Ameren Energy Generating Company [Member]	Dec. 31, 2011 Voluntary Separation Offer [Member]	Dec. 31, 2011 SO2 Emission Allowances [Member] Merchant Generation [Member]	Dec. 31, 2011 SO2 Emission Allowances [Member] Ameren Missouri [Member]	Dec. 31, 2011 Minimum [Member]	Dec. 31, 2010 Minimum [Member]	Dec. 31, 2009 Minimum [Member]	Dec. 31, 2011 Maximum [Member]	Dec. 31, 2010 Maximum [Member]	Dec. 31, 2009 Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Pretax impairment charge	$ 2,000,000	[1]	$ 68,000,000	[1]													$ 2,000,000	$ 1,000,000
Impairment charge on goodwill			420,000,000	[1],[2]
Book value	7,000,000		7,000,000			7,000,000		3,000,000
Unrecognized tax benefits that would impact effective tax rate	1,000,000				6,000,000
Percentage of EEI not owned by Ameren	20.00%
Percent of property sold											25.00%
Proceeds from sale										45,000,000	18,000,000				4,000,000
Pretax gain recognized on sale										8,000,000	5,000,000				4,000,000
Percent of average depreciable cost																			3.00%	3.00%	3.00%	4.00%	4.00%	4.00%
Proceeds from sales of properties	53,000,000		27,000,000		2,000,000		49,000,000	18,000,000	16,000,000
Additional contingent proceeds from sale of properties									1,000,000
Pretax charge to earnings in connection with the retirement of two generating units at its Meredosia power plant and for related obsolete inventory					4,000,000
Number of employee positions eliminated												90		300		340
Severance costs												$ 4,000,000	$ 4,000,000	$ 17,000,000		$ 28,000,000

[1] Includes amounts for registrant and nonregistrant subsidiaries.

[2] Includes amounts for Ameren registrants and nonregistrant subsidiaries.