Other Income and Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 8	[1]	$ 8	[1]	$ 16	[1]	$ 17	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]	14	[1]	14	[1]
Interest and dividend income	1	[1]	4	[1]	1	[1]	4	[1]
Other	0	[1]	0	[1]	0	[1]	1	[1]
Total miscellaneous income	16	[1]	19	[1]	31	[1]	36	[1]
Donations	1.0	[1]	3.0	[1]	5.0	[1]	15.0	[1],[2]
Other	4	[1]	4	[1]	8	[1]	7	[1]
Total miscellaneous expense	5	[1]	7	[1]	13	[1]	22	[1]
Ameren Missouri [Member]
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	7		7		14		15
Interest income on industrial development revenue bonds	7		7		14		14
Interest and dividend income	0		4		0		4
Total miscellaneous income	14		18		28		33
Donations	1.0		3.0		3.0		5.0
Other	2		1		5		2
Total miscellaneous expense	3		4		8		7
Ameren Illinois [Member]
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1		1		2		2
Interest and dividend income	1		0		1		0
Other	0		1		0		1
Total miscellaneous income	2		2		3		3
Donations	0		0		3.0		10.0	[2]
Other	1		2		1		3
Total miscellaneous expense	1		2		4		13
Ameren Illinois [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 election to participate in the formula ratemaking process.