Derivative Financial Instruments (Credit Risk) (Details) $ in Millions	6 Months Ended
Jun. 30, 2021 USD ($)
Credit Derivatives [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	$ 61
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	57
Ameren Illinois Company
Credit Derivatives [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	36
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	36
Union Electric Company
Credit Derivatives [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	25
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	$ 21

Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611

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- Definition

Maximum potential amount of future undiscounted payments that could be required under the credit derivative, before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4K
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5708775-113959

+ Details

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- Definition

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.

+ References

No definition available.

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- Details

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- Details

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