Other Income And Expenses (Other Income And Expenses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Allowance for equity funds used during construction	$ 9	[1]	$ 6	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]
Interest and dividend income			1	[1]
Other	1	[1]	2	[1]
Total miscellaneous income	17	[1]	16	[1]
Donations	12.0	[1],[2]	2.0	[1],[2]
Other	3	[1]	3	[1]
Total miscellaneous expense	15	[1]	5	[1]
Ameren Illinois Company [Member]
Total miscellaneous income	1		2
Total miscellaneous expense	11		1
Customer Assistance Programs [Member] | Ameren Illinois Company [Member]
Donations	1.0
One-Time Donation [Member] | Illinois Science and Energy Innovation Trust [Member] | Ameren Illinois Company [Member]
Donations	7.5
Annual Donation [Member] | Illinois Science and Energy Innovation Trust [Member] | Ameren Illinois Company [Member]
Donations	$ 1.0

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

[2] Includes Ameren Illinois' one-time $7.5 million donation and $1 million annual donation to the Illinois Science and Energy Innovation Trust and $1 million annual donation for customer assistance programs pursuant to the IEIMA as a result of Ameren Illinois' participation in the formula ratemaking process.