Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 23	$ 22
Cash Surrender Value of Life Insurance	258	244
Corporate owned life insurance, borrowings	113	113
Unrecorded Unconditional Purchase Obligation	1,755
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Cash Surrender Value of Life Insurance	123	122
Corporate owned life insurance, borrowings	113	$ 113
Unrecorded Unconditional Purchase Obligation	526
Variable Interest Entity, Not Primary Beneficiary [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	16
Partnership Funding Commitment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 15

Basis Of Presentation And Significant Accounting Policies [Line Items]

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Corporate owned life insurance, borrowings

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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.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
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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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The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Net carrying amount of the assets and liabilities in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), when the reporting entity is not the VIE's primary beneficiary.

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