Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 6	$ 7	$ 16	$ 20
Interest income on industrial development revenue bonds	[1]	7	7	20	20
Interest income	[1]	0	3	5	11
Other	[1]	0	1	1	3
Total miscellaneous income	[1]	13	18	42	54
Donations	[1]	0	1	7	8
Other	[1]	2	7	9	13
Total miscellaneous expense	[1]	2	8	16	21
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		6	6	15	16
Interest income on industrial development revenue bonds		7	7	20	20
Interest income		0	1	0	1
Other		0	0	1	1
Total miscellaneous income		13	14	36	38
Donations		0	0	2	2
Other		2	2	4	4
Total miscellaneous expense		2	2	6	6
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		0	1	1	4
Interest income		1	2	5	9
Other		0	1	1	2
Total miscellaneous income		1	4	7	15
Donations		0	1	5	6
Other		0	2	3	5
Total miscellaneous expense		$ 0	$ 3	$ 8	$ 11

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