Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 4	$ 5	$ 10	$ 13
Interest income on industrial development revenue bonds	[1]	6	6	13	13
Interest income	[1]	3	4	5	8
Other	[1]	1	1	1	2
Total miscellaneous income	[1]	14	16	29	36
Donations	[1]	2	2	7	7
Other	[1]	3	4	7	6
Total miscellaneous expense	[1]	5	6	14	13
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		4	3	9	10
Interest income on industrial development revenue bonds		6	6	13	13
Other		1	0	1	1
Total miscellaneous income		11	9	23	24
Donations		2	1	2	2
Other		0	1	2	2
Total miscellaneous expense		2	2	4	4
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		0	2	1	3
Interest income		2	3	4	7
Other		1	1	1	1
Total miscellaneous income		3	6	6	11
Donations		1	1	5	5
Other		1	2	3	3
Total miscellaneous expense		$ 2	$ 3	$ 8	$ 8

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