Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 8	$ 9	$ 13	$ 14
Interest income on industrial development revenue bonds	6	7	13	13
Interest income	3	2	4	4
Non-service cost components of net periodic benefit income	22	19	44	35
Other income	1	2	4	3
Donations	(1)	(6)	(7)	(11)
Other expense	(3)	(4)	(6)	(6)
Other Income (Expense), Net	36	29	65	52
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	5	7	8	11
Interest income on industrial development revenue bonds	6	7	13	13
Interest income	0	1	0	1
Non-service cost components of net periodic benefit income	4	4	9	9
Other income	2	0	2	1
Donations	0	(2)	(2)	(3)
Other expense	(1)	(1)	(2)	(3)
Other Income (Expense), Net	16	16	28	29
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	8	4	15	8
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	2	5	3
Interest income	2	1	4	3
Non-service cost components of net periodic benefit income	12	10	24	17
Other income	1	2	2	2
Donations	(1)	(1)	(5)	(5)
Other expense	(2)	(1)	(4)	(1)
Other Income (Expense), Net	$ 15	$ 13	$ 26	$ 19