Other Income And Expenses (Other Income And Expenses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Allowance for equity funds used during construction	$ 8	[1]	$ 9	[1]	$ 17	[1]	$ 15	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]	14	[1]	14	[1]
Interest and dividend income	5	[1],[2]	1	[1],[2]	5	[1],[2]	2	[1],[2]
Other					1	[1]	2	[1]
Total miscellaneous income	20	[1]	17	[1]	37	[1]	33	[1]
Donations	3.0	[1],[3]	1.0	[1],[3]	15.0	[1],[3]	3.0	[1],[3]
Other	4	[1]	4	[1]	7	[1]	7	[1]
Total miscellaneous expense	7	[1]	5	[1]	22	[1]	10	[1]
Ameren Missouri [Member]
Allowance for equity funds used during construction	7		8		15		14
Interest income on industrial development revenue bonds	7		7		14		14
Interest and dividend income	4	[2]	1	[2]	4	[2]	1	[2]
Total miscellaneous income	18		16		33		29
Donations	3.0		1.0		5.0		2.0
Other	1		2		2		4
Total miscellaneous expense	4		3		7		6
Ameren Illinois Company [Member]
Interest and dividend income							1
Other	1				1		1
Total miscellaneous income	2		1		3		3
Donations					10.0	[3]
Other	2		1		3		2
Total miscellaneous expense	2		1		13		2
Allowance for equity funds used during construction	1		1		2		1
Ameren Illinois Company [Member] | One-Time Donation [Member] | Illinois Science And Energy Innovation Trust [Member]
Donations					7.5
Ameren Illinois Company [Member] | Annual Donation [Member] | Illinois Science And Energy Innovation Trust [Member]
Donations					1.0
Ameren Illinois Company [Member] | Annual Donation [Member] | Customer Assistance Programs [Member]
Donations					$ 1.0

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

[2] Includes interest income relating to a refund of charges included in an expired power purchase agreement with Entergy. See Note 2 - Rate and Regulatory Matters for additional information.

[3] 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.