Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 8	$ 10	$ 19	$ 26
Interest income on industrial development revenue bonds	[1]	7	6	20	20
Interest and dividend income	[1]	4	3	12	8
Other	[1]	0	2	3	6
Total miscellaneous income	[1]	19	21	54	60
Donations	[1]	0	3	10	9
Other	[1]	5	4	12	11
Total miscellaneous expense	[1]	5	7	22	20
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		7	9	16	24
Interest income on industrial development revenue bonds		7	6	20	20
Interest and dividend income		0	0	1	1
Total miscellaneous income		14	15	37	45
Donations		0	2	3	5
Other		3	2	5	5
Total miscellaneous expense		3	4	8	10
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		1	1	3	2
Interest and dividend income		3	2	10	5
Other		0	1	2	5
Total miscellaneous income		4	4	15	12
Donations		0	0	4	3
Other		3	2	6	4
Total miscellaneous expense		$ 3	$ 2	$ 10	$ 7

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