Other Income and Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 10	[1]	$ 9	[1]	$ 26	[1]	$ 26	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]	21	[1]	21	[1]
Interest and dividend income	2	[1]	0	[1]	3	[1]	4	[1]
Other	1	[1]	1	[1]	1	[1]	2	[1]
Total miscellaneous income	20	[1]	17	[1]	51	[1]	53	[1]
Donations	2.0	[1]	3.0	[1]	7.0	[1]	18.0	[1],[2]
Other	3	[1]	3	[1]	11	[1]	10	[1]
Total miscellaneous expense	5	[1]	6	[1]	18	[1]	28	[1]
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	8		8		22		23
Interest income on industrial development revenue bonds	7		7		21		21
Interest and dividend income	1		0		1		4
Total miscellaneous income	16		15		44		48
Donations	0		2.0		3.0		7.0
Other	2		2		7		4
Total miscellaneous expense	2		4		10		11
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2		1		4		3
Interest and dividend income	1		0		2		0
Other	1		1		1		2
Total miscellaneous income	4		2		7		5
Donations	0		1.0		3.0		11.0	[2]
Other	3		1		4		4
Total miscellaneous expense	3		2		7		15
Ameren Illinois Company | 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 contribution to the Illinois Science and Energy Innovation Trust pursuant to the IEIMA as a result of Ameren Illinois’ 2012 election to participate in the formula ratemaking process.