Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 7	$ 8	$ 20	$ 19
Interest income on industrial development revenue bonds	[1]	7	7	20	20
Interest income	[1]	3	4	11	12
Other	[1]	1	0	3	3
Total miscellaneous income	[1]	18	19	54	54
Donations	[1]	1	0	8	10
Other	[1]	7	5	13	12
Total miscellaneous expense	[1]	8	5	21	22
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		6	7	16	16
Interest income on industrial development revenue bonds		7	7	20	20
Interest income		1	0	1	1
Other		0	0	1	0
Total miscellaneous income		14	14	38	37
Donations		0		2	3
Other		2	3	4	5
Total miscellaneous expense		2	3	6	8
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		1	1	4	3
Interest income		2	3	9	10
Other		1	0	2	2
Total miscellaneous income		4	4	15	15
Donations		1	0	6	4
Other		2	3	5	6
Total miscellaneous expense		$ 3	$ 3	$ 11	$ 10

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