Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 9	$ 8	$ 13	$ 13
Interest income on industrial development revenue bonds	6	6	12	13
Other interest income	1	3	2	4
Non-service cost components of net periodic benefit income	30	22	53	44
Miscellaneous income	7	1	9	4
Donations	(1)	(1)	(14)	(7)
Miscellaneous expense	(4)	(3)	(6)	(6)
Total Other Income, Net	48	36	69	65
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	6	5	8	8
Interest income on industrial development revenue bonds	7	6	12	13
Non-service cost components of net periodic benefit income	14	4	19	9
Miscellaneous income	1	2	2	2
Donations	(1)	0	(9)	(2)
Miscellaneous expense	(2)	(1)	(3)	(2)
Total Other Income, Net	25	16	29	28
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(3)	8	3	15
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	3	5	5
Other interest income	1	2	2	4
Non-service cost components of net periodic benefit income	11	12	24	24
Miscellaneous income	4	1	5	2
Donations	(1)	(1)	(5)	(5)
Miscellaneous expense	(1)	(2)	(3)	(4)
Total Other Income, Net	17	$ 15	28	$ 26
Final Rate Order | Electricity | Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations	$ (8)		$ (8)