Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Nov. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 189	$ 164
Reduction to FERC allowed base return on common equity		0.50%
Incentive adder cap on FERC allowed return on equity		2.50%
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 90	$ 84
Net Income (Loss) Attributable to Parent		120		$ 120
February 2015 FERC ROE Complaint Case
Rate And Regulatory Matters [Line Items]
Net Income (Loss) Attributable to Parent		11
February 2015 FERC ROE Complaint Case | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Net Income (Loss) Attributable to Parent		$ 6
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%	10.32%	9.88%
Customer Requested Rate on Equity		9.15%
Current regulatory liabilities		$ 40
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		$ 23
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%