Other Income and Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 5	[1]	$ 7	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]
Interest and dividend income	4	[1]	2	[1]
Other	3	[1]	2	[1]
Total miscellaneous income	19	[1]	18	[1]
Donations	8	[1]	5	[1]
Other	3	[1]	4	[1]
Total miscellaneous expense	11	[1]	9	[1]
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	4		7
Interest income on industrial development revenue bonds	7		7
Total miscellaneous income	11		14
Donations	2		2
Other	1		2
Total miscellaneous expense	3		4
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1
Interest and dividend income	4		2
Other	2		1
Total miscellaneous income	7		3
Donations	3		3
Other	2		1
Total miscellaneous expense	$ 5		$ 4

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