Rate And Regulatory Matters (Narrative-Illinois) (Details) - Ameren Illinois Company - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Public Utilities, General Disclosures [Line Items]
Deferred payment arrangement extension		24 months
Financial assistance program costs		$ 8
Final Rate Order
Public Utilities, General Disclosures [Line Items]
Electric Energy-Efficiency Revenue Requirement		$ 51
QIP rider
Public Utilities, General Disclosures [Line Items]
QIP rider reset		zero
IEIMA revenue requirement reconciliation adjustment \| Final Rate Order \| Electric Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 49
IEIMA revenue requirement reconciliation adjustment \| Pending Rate Case \| Electric Distribution
Public Utilities, General Disclosures [Line Items]
Requested electric distribution service rate reconciliation period		2 years
FEJA energy-efficiency rider \| Final Rate Order
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 7
Subsequent Event \| Pending Rate Case \| Natural gas
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 76
Public Utilities, Requested Return on Equity, Percentage	9.67%
Public Utilities, Requested Equity Capital Structure, Percentage	52.00%
Rate Base	$ 2,100

Number of months by which deferred payment arrangements were extended.

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Revenue requirement for the electric energy-efficiency investment

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Costs incurred related to financial assistance programs.

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- Definition

When QIP rate base is transferred to base rates upon the issuance of a natural gas delivery service rate order, the QIP rider is reset to zero.

+ References

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- Definition

Value established for a utility on which the entity is allowed to earn a particular rate of return

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- Definition

The requested period to reconcile electric distribution service rates after formula rates have end or are no longer elected.

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Amount of public utility's approved rate increase (decrease) by regulatory agency.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage of public utility's requested equity to capital, including debt and equity.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested return on equity.

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