Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 9	$ 4	$ 14	$ 10
Interest income on industrial development revenue bonds	7	6	13	13
Interest income	2	3	4	5
Non-service cost components of net periodic benefit income	19	10	35	22
Other income	2	2	3	2
Donations	(6)	(2)	(11)	(7)
Other expense	(4)	(3)	(6)	(7)
Other Income (Expense), Net	29	20	52	38
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	7	4	11	9
Interest income on industrial development revenue bonds	7	6	13	13
Interest income	1	1	1	1
Non-service cost components of net periodic benefit income	4	6	9	12
Other income	0	1	1	1
Donations	(2)	(2)	(3)	2
Other expense	(1)	0	(3)	(2)
Other Income (Expense), Net	16	16	29	32
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	4		8
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2	0	3	1
Interest income	1	2	3	4
Non-service cost components of net periodic benefit income	10	1	17	4
Other income	2	2	2	2
Donations	(1)	(1)	(5)	(5)
Other expense	(1)	(1)	(1)	(3)
Other Income (Expense), Net	$ 13	$ 3	$ 19	$ 3