Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2019
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	26.00%		17.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 116		$ 83
Noncontrolling Interest in Variable Interest Entity	48	$ 37
Cash Surrender Value of Life Insurance	279	272
Corporate owned life insurance, borrowings	$ 112	107
Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	17.00%		11.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 32		$ 24
Union Electric Company \| Final Rate Order
Basis Of Presentation And Significant Accounting Policies [Line Items]
COVID-19 pandemic costs	$ 9
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement	35.00%		25.00%
Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement	$ 84		$ 59
Cash Surrender Value of Life Insurance	119	115
Corporate owned life insurance, borrowings	112	$ 107
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 30

Accounts receivable balances that are 30 days or more past due or part of a deferred payment arrangement

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Basis Of Presentation And Significant Accounting Policies [Line Items]

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Costs incurred related to the COVID-19 pandemic.

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Corporate owned life insurance, borrowings

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Percentage of accounts receivable balances that are 30 days past due or are part of a deferred payment arrangement

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- Definition

Carrying amount as of the balance sheet date of amounts which could be received based on the terms of the insurance contract upon surrendering life policies owned by the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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Portion of equity (net assets) in a variable interest entity (VIE) not attributable, directly or indirectly, to the parent entity. That is, this is the portion of equity in a VIE that is attributable to the noncontrolling interest (previously referred to as minority interest).

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Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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-Topic 440
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