Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 7	$ 11	$ 20	$ 25
Interest income on industrial development revenue bonds	6	6	19	19
Other interest income	2	2	6	6
Non-service cost components of net periodic benefit income	23	17	67	52
Miscellaneous income	2	2	6	5
Donations	(1)	(4)	(8)	(15)
Miscellaneous expense	(5)	(2)	(11)	(8)
Total Other Income, Net	34	32	99	84
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	6	8	14	19
Interest income on industrial development revenue bonds	6	6	19	19
Other interest income	0	1	0	2
Non-service cost components of net periodic benefit income	4	4	13	13
Miscellaneous income	2	2	4	3
Donations	(1)	(3)	(3)	(6)
Miscellaneous expense	(2)	(2)	(4)	(5)
Total Other Income, Net	15	16	43	45
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	7	5	22	13
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1	3	6	6
Other interest income	1	1	5	4
Non-service cost components of net periodic benefit income	12	8	36	25
Miscellaneous income	1	1	3	3
Donations	0	0	(5)	(5)
Miscellaneous expense	(2)	(2)	(6)	(3)
Total Other Income, Net	$ 13	$ 11	$ 39	$ 30