Other Income And Expenses (Other Income And Expenses) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Other Nonoperating Income (Expense) [Line Items]
Interest and dividend income	$ 5	[1],[2]	$ 4	[1]	$ 5	[1]
Interest income on industrial development revenue bonds	28	[1]	28	[1]	28	[1]
Allowance for equity funds used during construction	36	[1]	34	[1]	52	[1]
Other	2	[1]	3	[1]	5	[1]
Total miscellaneous income	71	[1]	69	[1]	90	[1]
Donations	24	[1],[3]	8	[1]	19	[1]
Other	13	[1]	15	[1]	14	[1]
Total miscellaneous expense	37	[1]	23	[1]	33	[1]
Union Electric Company [Member]
Other Nonoperating Income (Expense) [Line Items]
Interest and dividend income	4	[2]	2		3
Interest income on industrial development revenue bonds	28		28		28
Allowance for equity funds used during construction	31		30		50
Other			1		2
Total miscellaneous income	63		61		83
Donations	9		3		8
Other	5		7		5
Total miscellaneous expense	14		10		13
Ameren Illinois Company [Member]
Other Nonoperating Income (Expense) [Line Items]
Interest and dividend income			1		1
Interest income on industrial development revenue bonds	5		4		2
Allowance for equity funds used during construction	2		2		4
Total miscellaneous income	7		7		7
Donations	11	[3]	1		5
Other	6		5		8
Total miscellaneous expense	17		6		13
Illinois Science and Energy Innovation Trust [Member] | Ameren Illinois Company [Member]
Other Nonoperating Income (Expense) [Line Items]
One-time Donation	7.5
Annual Donation	1
Customer Assistance Programs [Member] | Ameren Illinois Company [Member]
Other Nonoperating Income (Expense) [Line Items]
Annual Donation	$ 1

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

[2] Includes interest income relating to a 2012 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' 2012 participation in the formula ratemaking process.