Rate And Regulatory Matters (Narrative-Illinois) (Detail) - Ameren Illinois Company - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2021	Mar. 31, 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
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		two years
Subsequent Event | Pending Rate Case | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 64