Rate And Regulatory Matters (Narrative-Illinois) (Detail) - Ameren Illinois Company - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Sep. 30, 2020	Jun. 30, 2020
Rate And Regulatory Matters [Line Items]
Deferred payment arrangement extension		24 months
Pending Rate Case | Natural Gas
Rate And Regulatory Matters [Line Items]
ICC Staff recommended rate increase (decrease)		$ 67
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
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)		53
Pending Rate Case | FEJA energy-efficiency rider [Domain] | 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]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 96
Revenues	$ 46
Public Utilities, Requested Return on Equity, Percentage	10.50%
Public Utilities, Requested Equity Capital Structure, Percentage	54.10%
Rate Base	$ 2,100