Rate And Regulatory Matters (Narrative-Illinois) (Detail) - Ameren Illinois Company - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Sep. 30, 2021	Jun. 30, 2021
Rate And Regulatory Matters [Line Items]
Annual investment in energy-efficiency programs		$ 100
Deferred payment arrangement extension		18 months
Financial assistance program costs		$ 4
Pending Rate Case | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
ICC Staff recommended rate increase (decrease)		$ 54
Regulatory Asset, Amortization Period		5 years
Pending Rate Case | FEJA energy-efficiency rider | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 11
IEIMA
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage		5.80%
Final Rate Order | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
Requested electric distribution service rate reconciliation period		2 years
Amortization Period		2 years
Subsequent Event | Pending Rate Case | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 60