Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 6	$ 5
Interest income on industrial development revenue bonds	6	6
Interest income	2	2
Non-service cost components of net periodic benefit income	22	16
Other income	2	1
Donations	(6)	(5)
Other expense	(3)	(2)
Other Income (Expense), Net	29	23
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	4	4
Interest income on industrial development revenue bonds	6	6
Non-service cost components of net periodic benefit income	4	5
Other income	1	1
Donations	(2)	(1)
Other expense	(1)	(2)
Other Income (Expense), Net	12	13
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	7	4
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2	1
Interest income	2	2
Non-service cost components of net periodic benefit income	12	7
Other income	1	0
Donations	(4)	(4)
Other expense	(2)	0
Other Income (Expense), Net	$ 11	$ 6