Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 5	$ 6	$ 13	$ 11
Interest income on industrial development revenue bonds	[1]	6	6	13	13
Interest and dividend income	[1]	4	4	8	8
Other	[1]	1	0	2	3
Total miscellaneous income	[1]	16	16	36	35
Donations	[1]	2	2	7	10
Other	[1]	4	4	6	7
Total miscellaneous expense	[1]	6	6	13	17
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		3	5	10	9
Interest income on industrial development revenue bonds		6	6	13	13
Interest and dividend income		0	1	0	1
Other		0	0	1	0
Total miscellaneous income		9	12	24	23
Donations		1	1	2	3
Other		1	1	2	2
Total miscellaneous expense		2	2	4	5
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		2	1	3	2
Interest and dividend income		3	3	7	7
Other		1	0	1	2
Total miscellaneous income		6	4	11	11
Donations		1	1	5	4
Other		2	1	3	3
Total miscellaneous expense		$ 3	$ 2	$ 8	$ 7

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