Rate And Regulatory Matters (Narrative-Illinois) (Detail) - Ameren Illinois Company - USD ($) $ in Millions	1 Months Ended				3 Months Ended	9 Months Ended
	Oct. 31, 2020	Sep. 30, 2020	May 31, 2020	Apr. 30, 2020	Sep. 30, 2020	Sep. 30, 2020
Rate And Regulatory Matters [Line Items]
Deferred payment arrangement extension						24 months
Financial assistance program costs						$ 8
Pending Rate Case | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 97
Revenues					$ 46
Public Utilities, Requested Return on Equity, Percentage					10.50%
Public Utilities, Requested Equity Capital Structure, Percentage					54.10%
Rate Base					$ 2,100
Pending Rate Case | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 45
ICC Staff recommended rate increase (decrease)		$ (49)
Pending Rate Case | FEJA energy-efficiency rider | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 7
Subsequent Event | Pending Rate Case | Natural Gas
Rate And Regulatory Matters [Line Items]
ICC Staff recommended rate increase (decrease)	$ 69
ICC Staff recommended Return on Equity, Percentage	9.30%
ICC Staff recommended Equity Capital Structure, Percentage	50.40%
ICC Staff recommended Rate Base	$ 2,100