Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 12	$ 7	$ 25	$ 20
Interest income on industrial development revenue bonds	7	6	19	19
Other interest income	0	2	2	6
Non-service cost components of net periodic benefit income	32	23	85	67
Miscellaneous income	1	2	10	6
Donations	0	(1)	(14)	(8)
Miscellaneous expense	(4)	(5)	(10)	(11)
Total Other Income, Net	48	34	117	99
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	7	6	15	14
Interest income on industrial development revenue bonds	7	6	19	19
Non-service cost components of net periodic benefit income	13	4	32	13
Miscellaneous income	1	2	3	4
Donations	0	(1)	(9)	(3)
Miscellaneous expense	(2)	(2)	(5)	(4)
Total Other Income, Net	26	15	55	43
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(3)	7	(1)	22
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	5	1	10	6
Other interest income	0	1	2	5
Non-service cost components of net periodic benefit income	12	12	36	36
Miscellaneous income	1	1	6	3
Donations	0	0	(5)	(5)
Miscellaneous expense	(1)	(2)	(4)	(6)
Total Other Income, Net	$ 17	$ 13	45	$ 39
Final Rate Order | Electricity | Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations			$ (8)