Other Income and Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 8	[1]	$ 9	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]
Other	0	[1]	1	[1]
Total miscellaneous income	15	[1]	17	[1]
Donations	4.0	[1]	12.0	[1],[2]
Other	4	[1]	3	[1]
Total miscellaneous expense	8	[1]	15	[1]
Union Electric Company [Member]
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	7		8
Interest income on industrial development revenue bonds	7		7
Total miscellaneous income	14		15
Donations	2.0		2.0
Other	3		1
Total miscellaneous expense	5		3
Ameren Illinois Company [Member]
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1		1
Total miscellaneous income	1		1
Donations	3.0		10.0	[2]
Other	0		1
Total miscellaneous expense	3		11
Ameren Illinois Company [Member] | One-Time Donation [Member] | Illinois Science And Energy Innovation Trust [Member]
Other Nonoperating Income (Expense) [Line Items]
Donations	$ 7.5

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

[2] Includes Ameren Illinois’ one-time $7.5 million donation to the Illinois Science and Energy Innovation Trust pursuant to the IEIMA as a result of Ameren Illinois’ 2012 participation in the formula ratemaking process.