Other Income and Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 10	[1]	$ 10	[1]	$ 26	[1]	$ 26	[1]
Interest income on industrial development revenue bonds	6	[1]	7	[1]	20	[1]	21	[1]
Interest and dividend income	3	[1]	2	[1]	8	[1]	3	[1]
Other	2	[1]	1	[1]	6	[1]	1	[1]
Total miscellaneous income	21	[1]	20	[1]	60	[1]	51	[1]
Donations	3	[1]	2	[1]	9	[1]	7	[1]
Other	4	[1]	3	[1]	11	[1]	11	[1]
Total miscellaneous expense	7	[1]	5	[1]	20	[1]	18	[1]
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	9		8		24		22
Interest income on industrial development revenue bonds	6		7		20		21
Interest and dividend income			1		1		1
Total miscellaneous income	15		16		45		44
Donations	2				5		3
Other	2		2		5		7
Total miscellaneous expense	4		2		10		10
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1		2		2		4
Interest and dividend income	2		1		5		2
Other	1		1		5		1
Total miscellaneous income	4		4		12		7
Donations					3		3
Other	2		3		4		4
Total miscellaneous expense	$ 2		$ 3		$ 7		$ 7

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