Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 9	$ 9	$ 16	$ 13
Interest income on industrial development revenue bonds	6	6	12	12
Other interest income	0	1	1	2
Non-service cost components of net periodic benefit income	34	30	68	53
Miscellaneous income	7	7	11	9
Donations	(1)	(1)	(4)	(14)
Miscellaneous expense	(6)	(4)	(9)	(6)
Total Other Income, Net	49	48	95	69
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(3)	(3)	(3)	3
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	6	6	10	8
Interest income on industrial development revenue bonds	6	7	12	12
Non-service cost components of net periodic benefit income	14	14	28	19
Miscellaneous income	0	1	1	2
Donations	(1)	(1)	(1)	(9)
Miscellaneous expense	(1)	(2)	(3)	(3)
Total Other Income, Net	24	25	47	29
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	(3)	(3)	(3)	3
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	3	6	5
Other interest income	0	1	1	2
Non-service cost components of net periodic benefit income	14	11	28	24
Miscellaneous income	3	4	3	5
Donations	0	(1)	(3)	(5)
Miscellaneous expense	(4)	(1)	(5)	(3)
Total Other Income, Net	$ 16	$ 17	$ 30	28
Final Rate Order | Electricity | Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Donations				$ (8)