Rate and Regulatory Matters (Narrative-Federal) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	$ 164	$ 164	$ 149
Reduction to FERC allowed base return on common equity		0.50%
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	$ 84	$ 84	62
Net Income Available to Common Shareholder		$ 343	$ 304	$ 268
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	9.88%	10.32%
Customer Requested Rate on Equity		9.15%
Current regulatory liabilities	$ 40	$ 40
Incentive adder to FERC allowed base return on common equity		0.50%
Final Rate Order | Midwest Independent Transmission System Operator, Inc | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	23	$ 23
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%
February 2015 FERC ROE Complaint Case
Public Utilities, General Disclosures [Line Items]
Net Income Available to Common Shareholder		$ 10
February 2015 FERC ROE Complaint Case | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Net Income Available to Common Shareholder		6
February 2015 FERC ROE Complaint Case | Midwest Independent Transmission System Operator, Inc
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	46	46
February 2015 FERC ROE Complaint Case | Midwest Independent Transmission System Operator, Inc | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	$ 27	$ 27