Other Income and Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 7	[1]	$ 8	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]
Interest and dividend income	2	[1]		[1]
Other	2	[1]		[1]
Total miscellaneous income	18	[1]	15	[1]
Donations	5	[1]	4	[1]
Other	4	[1]	4	[1]
Total miscellaneous expense	9	[1]	8	[1]
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	7		7
Interest income on industrial development revenue bonds	7		7
Total miscellaneous income	14		14
Donations	2		2
Other	2		3
Total miscellaneous expense	4		5
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction			1
Interest and dividend income	2
Other	1
Total miscellaneous income	3		1
Donations	3		3
Other	1
Total miscellaneous expense	$ 4		$ 3

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