Rate and Regulatory Matters (Narrative-Federal) (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	May 31, 2020	Nov. 30, 2019	Dec. 31, 2020	Dec. 31, 2019
Public Utilities, General Disclosures [Line Items]
Lower revenue due to transmission formula rate revisions			$ 2
Income recognized due to higher FERC-allowed base ROE			13
Current regulatory liabilities			121	$ 164
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Income recognized due to higher FERC-allowed base ROE			7
Current regulatory liabilities			$ 88	$ 84
Midwest Independent Transmission System Operator, Inc
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage			12.38%
Incentive adder to FERC allowed base return on common equity			0.50%
Final Rate Order | Midwest Independent Transmission System Operator, Inc
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved 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			$ 15
Final Rate Order | Midwest Independent Transmission System Operator, Inc | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities			$ 7
Final Rate Order | Midwest Independent Transmission System Operator, Inc | Maximum
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage			10.82%