Goodwill, Impairment And Other Charges (Narrative) (Details) (USD $)	12 Months Ended																	1 Months Ended		12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009		Dec. 31, 2011 Ameren Energy Generating Company [Member]	Dec. 31, 2010 Ameren Energy Generating Company [Member]	Dec. 31, 2009 Ameren Energy Generating Company [Member]		Dec. 31, 2009 Rail Line Extension Project [Member]	Dec. 31, 2009 Indian Trails Facility [Member]	Dec. 31, 2011 Taum Sauk Energy Center [Member]	Dec. 31, 2011 Meredosia And Hutsonville [Member]	Dec. 31, 2011 Closure of Meredosia and Hutsonville Energy Centers [Member]	Dec. 31, 2011 SO2 Emission Allowances [Member]	Dec. 31, 2010 SO2 Emission Allowances [Member]	Jul. 31, 2011 SO2 Emission Allowances [Member] Ameren Missouri [Member]	Jul. 31, 2010 SO2 Emission Allowances [Member] Ameren Missouri [Member]	Dec. 31, 2011 Facility Closing [Member] Closure of Meredosia and Hutsonville Energy Centers [Member]	Dec. 31, 2010 Merchant Generation [Member]	Dec. 31, 2010 Merchant Generation [Member] Ameren Energy Generating Company [Member]
Impairment charge on goodwill			$ 420,000,000	[1],[2]																	$ 420,000,000
Number of employee positions eliminated														90
Noncash impairment charge	125,000,000	[1]	589,000,000	[1]	7,000,000	[1]	35,000,000	170,000,000	6,000,000	[3]												65,000,000
Non-cash impairment of materials and supplies					4,000,000										4,000,000
Severance costs															4,000,000
Loss from regulatory disallowance													89,000,000
Impairment charge on long-lived assets and related charges	123,000,000	[1]	101,000,000	[1]	7,000,000	[1]					6,000,000	1,000,000								26,000,000
Pretax impairment charge	2,000,000	[1]	68,000,000	[1]												2,000,000	68,000,000	1,000,000	23,000,000
Expected tax benefits related to closure of plants														22,000,000
Asset retirement obligations, noncurrent	428,000,000		475,000,000				66,000,000	74,000,000							38,000,000
Expected tax benefits related to asset retirement obligations															$ 16,000,000

[1] Includes amounts for registrant and nonregistrant subsidiaries.

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

[3] Prior period has been adjusted to include EEI as discussed in Note 1 - Summary of Significant Accounting Policies.