Rate And Regulatory Matters (Narrative-Illinois) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Estimated Capital Project Costs	$ 81
Property, Plant and Equipment, Additions	$ 39
Pending Rate Case | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	8.90%
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%
Electric Energy-Efficiency Investment	$ 205
Electric Energy-Efficiency Revenue Requirement	44
Pending Rate Case | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	7
Pending Rate Case | FEJA energy-efficiency rider | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 10