Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	May 31, 2020	Nov. 30, 2019	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Rate And Regulatory Matters [Line Items]
Lower revenue due to transmission formula rate revisions			$ 2
Income recognized due to higher FERC-allowed base ROE			13
Current regulatory liabilities				$ 96	$ 164
Incentive adder cap on FERC allowed return on equity				2.50%
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Income recognized due to higher FERC-allowed base ROE			$ 7
Current regulatory liabilities				$ 47	$ 84
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%	10.02%	10.32%
Customer Requested Rate on Equity				9.15%
Current regulatory liabilities				$ 15
Incentive adder to FERC allowed base return on common equity				0.50%
Midwest Independent Transmission System Operator, Inc | Final Rate Order | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities				$ 7
Midwest Independent Transmission System Operator, Inc | Notice of Proposed Rulemaking
Rate And Regulatory Matters [Line Items]
Incentive adder to FERC allowed base return on common equity				1.00%
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%