[Line Items] for Accounting Policies [Table]

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- Definition

Number or transmission line segments completed under the Illinois Rivers project.

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The resettlement of MISO-related purchase and sale transactions.

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Number of Months Between Callaway Maintenance Outages

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Provision For Depreciation Percent Of Average Depreciable Cost

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Public Utilities, Area Serviced

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Public Utilities, Number of Customers

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- Definition

Total transmission line segments under the Illinois Rivers project.

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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.

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- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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- Definition

Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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- Definition

Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Amount of increase (decrease) of physical assets used in the normal conduct of business and not intended for resale, from reclassification, impairment, donation, or changes classified as other. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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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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Summary Of Significant Accounting Policies (Narrative) (Details) customer in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019 USD ($) mi² segment customer	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Accounting Policies [Line Items]
MISO Resettlements		$ 0
Goodwill		$ 411,000,000	$ 411,000,000
Number of reportable segments \| segment		4
Goodwill, Impairment Loss		$ 0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net		28,000,000	22,000,000
Unrecorded Unconditional Purchase Obligation		1,886,000,000
Cash Surrender Value of Life Insurance		264,000,000	244,000,000
Loans, Gross, Insurance Policy		$ 114,000,000	$ 113,000,000
Ameren Missouri software licensing with Ameren Illinois [Member]			8
Total Transmission Line Segments - Illinois Rivers Project			9
Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced \| mi²		24,000
Unrecorded Unconditional Purchase Obligation		$ 1,310,000,000
Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced \| mi²		43,700
Goodwill		$ 411,000,000	$ 411,000,000
Number of reportable segments \| segment		3
Goodwill, Impairment Loss		$ 0
Unrecorded Unconditional Purchase Obligation		526,000,000
Cash Surrender Value of Life Insurance		$ 123,000,000	$ 122,000,000
Minimum
Accounting Policies [Line Items]
Percent of average depreciable cost		3.00%	3.00%	3.00%
Maximum
Accounting Policies [Line Items]
Percent of average depreciable cost		4.00%	4.00%	4.00%
Electricity \| Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers \| customer		1.2
Power \| Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers \| customer		1.2
Natural gas \| Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers \| customer		0.1
Natural gas \| Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers \| customer		0.8
Ameren Illinois Electric Distribution
Accounting Policies [Line Items]
Goodwill		$ 238,000,000
Ameren Illinois Gas
Accounting Policies [Line Items]
Goodwill		80,000,000
Ameren Illinois Transmission
Accounting Policies [Line Items]
Goodwill		93,000,000
Ameren Transmission
Accounting Policies [Line Items]
Goodwill		93,000,000
AROs \| Union Electric Company
Accounting Policies [Line Items]
Noncash Depreciation related to ARO		18,000,000	$ 14,000,000	$ 26,000,000
Subsequent Event \| Union Electric Company
Accounting Policies [Line Items]
Number of Months Between Callaway Maintenance Outages	18 months
Partnership Funding Commitment [Member]
Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation		35,000,000
Variable Interest Entity, Not Primary Beneficiary [Member]
Accounting Policies [Line Items]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		$ 13,000,000