Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 7	$ 4
Interest income on industrial development revenue bonds	6	6
Other interest income	1	1
Non-service cost components of net periodic benefit income	34	23
Miscellaneous income	4	2
Donations	(3)	(13)
Miscellaneous expense	(3)	(2)
Total Other Income, Net	46	21
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	4	2
Interest income on industrial development revenue bonds	6	6
Non-service cost components of net periodic benefit income	14	5
Miscellaneous income	1	1
Donations	0	(8)
Miscellaneous expense	(2)	(2)
Total Other Income, Net	23	4
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(1)	6
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	2
Other interest income	1	1
Non-service cost components of net periodic benefit income	14	13
Miscellaneous income	0	1
Donations	(3)	(4)
Miscellaneous expense	(1)	(2)
Total Other Income, Net	$ 14	11
Final Rate Order | Electricity | Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations		$ (8)