Rate And Regulatory Matters (Narrative-Illinois) (Details) - Ameren Illinois Company - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Final Rate Order
Public Utilities, General Disclosures [Line Items]
Electric Energy-Efficiency Revenue Requirement		$ 44
IEIMA revenue requirement reconciliation adjustment \| Final Rate Order \| Electric Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		7
FEJA energy-efficiency rider \| Final Rate Order
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 10
Subsequent Event \| Pending Rate Case \| Natural gas
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 102
Revenues	$ 46
Public Utilities, Requested Return on Equity, Percentage	10.50%
Public Utilities, Requested Equity Capital Structure, Percentage	54.10%
Rate Base	$ 2,100

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