[Line Items] for Accounting Policies [Table]

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- Definition

Asbestos trust fund balance

+ References

No definition available.

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- Definition

The non-service costs and income components of the net periodic benefit cost.

+ References

No definition available.

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- Definition

Percent of difference to be contributed to the asbestos trust fund if cash expenditures are less than amount included in base electric rates.

+ References

No definition available.

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- Definition

Premiums from Issuance of Debt

+ References

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- Definition

Provision For Depreciation Percent Of Average Depreciable Cost

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- Definition

Public Utilities, Area Serviced

+ References

No definition available.

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- Definition

Public Utilities, Number of Customers

+ References

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- Definition

Sharing Level For Fac

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No definition available.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
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-Name Accounting Standards Codification
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- Definition

Amount of cash outflow reducing loss contingency liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
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- Definition

Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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-Name Accounting Standards Codification
-Topic 740
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-Section 50
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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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- Definition

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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No definition available.

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Summary Of Significant Accounting Policies (Narrative) (Details) customer in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) mi² customer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Accounting Policies [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 9		$ 17	$ 9
Goodwill	411		411	411
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount			20
Loss Contingency Accrual, Payments					$ 25
Defined Benefit Plan, Non-service Cost or Income Components			$ 44	55
Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced \| mi²			40,000
Goodwill	411		$ 411	411
Defined Benefit Plan, Non-service Cost or Income Components			$ 10	24
Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced \| mi²			24,000
Defined Benefit Plan, Non-service Cost or Income Components			$ 22	$ 18
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%
Power \| Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers \| customer			1.2
Power \| Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers \| customer			1.2
Natural Gas \| Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers \| customer			0.8
Natural Gas \| Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers \| customer			0.1
FAC \| Union Electric Company
Accounting Policies [Line Items]
Sharing Level For Fac			95.00%
State
Accounting Policies [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities				$ 53
Asbestos-Related
Accounting Policies [Line Items]
Asbestos trust fund balance	$ 22		$ 23
Percent of difference to be contributed to the asbestos trust fund if cash expenditures are less than amount included in base electric rates.			90.00%
Asbestos-Related \| Ameren Illinois Company
Accounting Policies [Line Items]
Asbestos trust fund balance			$ 22
Ameren Illinois Electric Distribution [Member]
Accounting Policies [Line Items]
Goodwill			238
Ameren Illinois Gas [Member]
Accounting Policies [Line Items]
Goodwill			80
Ameren Illinois Transmission [Member]
Accounting Policies [Line Items]
Goodwill			93
Ameren Transmission [Member]
Accounting Policies [Line Items]
Goodwill			$ 93
Subsequent Event [Member]
Accounting Policies [Line Items]
Premiums from Issuance of Debt		$ 7