Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 4	$ 6
Interest income on industrial development revenue bonds	6	6
Other interest income	1	2
Non-service cost components of net periodic benefit income	23	22
Miscellaneous income	2	2
Donations	(13)	(6)
Miscellaneous expense	(2)	(3)
Total Other Income, Net	21	29
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2	4
Interest income on industrial development revenue bonds	6	6
Non-service cost components of net periodic benefit income	5	4
Miscellaneous income	1	1
Donations	(8)	(2)
Miscellaneous expense	(2)	(1)
Total Other Income, Net	4	12
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	6	7
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2	2
Other interest income	1	2
Non-service cost components of net periodic benefit income	13	12
Miscellaneous income	1	1
Donations	(4)	(4)
Miscellaneous expense	(2)	(2)
Total Other Income, Net	11	$ 11
Final Rate Order | Electricity | Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations	$ (8)