Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	May 31, 2020	Nov. 30, 2019	Mar. 31, 2021	Dec. 31, 2020
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			$ 5,230	$ 5,282
Current regulatory liabilities			225	121
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			1,988	2,063
Current regulatory liabilities			200	$ 88
Final Rate Order | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities			$ 9
Midwest Independent Transmission System Operator, Inc
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage			12.38%
Midwest Independent Transmission System Operator, Inc | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.02%	9.88%	10.32%
Customer Requested Rate on Equity			9.15%
Incentive adder to FERC allowed base return on common equity			0.50%
Current regulatory liabilities			$ 17
Midwest Independent Transmission System Operator, Inc | Final Rate Order | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities			$ 9
Maximum | Midwest Independent Transmission System Operator, Inc | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage			10.82%