Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 5	$ 6
Interest income on industrial development revenue bonds	6	7
Interest income	2	2
Non-service cost components of net periodic benefit income	16	12
Other income	1	0
Donations	(5)	(5)
Other expense	(2)	(4)
Other Income (Expense), Net	23	18
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	4	5
Interest income on industrial development revenue bonds	6	7
Non-service cost components of net periodic benefit income	5	6
Other income	1	0
Donations	(1)	0
Other expense	(2)	(2)
Other Income (Expense), Net	13	16
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	4
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1	1
Interest income	2	2
Non-service cost components of net periodic benefit income	7	3
Donations	(4)	(4)
Other expense	0	(2)
Other Income (Expense), Net	$ 6	$ 0