Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended
	Nov. 30, 2019	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Rate And Regulatory Matters [Line Items]
Income recognized due to higher FERC-allowed base ROE			$ 13
Current regulatory liabilities		$ 147	$ 147	$ 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		$ 70	$ 70	$ 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	9.88%	10.02%	10.32%
Customer Requested Rate on Equity			9.15%
Current regulatory liabilities		$ 23	$ 23
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		$ 11	$ 11
Midwest Independent Transmission System Operator, Inc | Notice of Proposed Rulemaking [Member]
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%