Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 11	$ 6	$ 25	$ 16
Interest income on industrial development revenue bonds	6	7	19	20
Interest income	2	0	6	5
Non-service cost components of net periodic benefit income	17	11	52	33
Other income	2	1	5	3
Donations	(4)	0	(15)	(7)
Other expense	(2)	(2)	(8)	(9)
Other Income (Expense), Net	32	23	84	61
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	8	6	19	15
Interest income on industrial development revenue bonds	6	7	19	20
Interest income	1	0	2	1
Non-service cost components of net periodic benefit income	4	5	13	17
Other income	2	0	3	1
Donations	(3)	0	(6)	2
Other expense	(2)	(2)	(5)	(4)
Other Income (Expense), Net	16	16	45	48
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	5		13
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	0	6	1
Interest income	1	1	4	5
Non-service cost components of net periodic benefit income	8	4	25	8
Other income	1	0	3	2
Donations	0	0	(5)	(5)
Other expense	(2)	0	(3)	(3)
Other Income (Expense), Net	$ 11	$ 5	$ 30	$ 8