The amount of expected costs to fund a capital project.

No definition available.

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- Definition

Ameren Illinois' electric energy efficiency investments under the FEJA.

+ References

No definition available.

+ Details

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- Definition

Increase Decrease in Return on Equity for Energy Savings Goals

+ References

No definition available.

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- Definition

Value established for a utility on which the entity is allowed to earn a particular rate of return

+ References

No definition available.

+ Details

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- Definition

Refunds to customers as a result of the reduction in the federal corporate income tax rate as a result of the Tax Cuts and Jobs Act.

+ References

No definition available.

+ Details

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- Definition

Amount of acquisition of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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- Definition

Amount of public utility's approved rate increase (decrease) by regulatory agency.

+ References

No definition available.

+ Details

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- Definition

Percentage of public utility's approved return on equity.

+ References

No definition available.

+ 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.

+ Details

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- Definition

Percentage of public utility's requested equity to capital, including debt and equity.

+ References

No definition available.

+ Details

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- Definition

Amount of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

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- Definition

Percentage of public utility's requested return on equity.

+ References

No definition available.

+ Details

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

+ Details

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Rate And Regulatory Matters (Narrative-Illinois) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Regulatory assets		$ 1,230	$ 1,437
Current regulatory assets		144	149
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets		822	816
Current regulatory assets		87	108
Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Estimated Capital Project Costs		85
Property, Plant and Equipment, Additions		$ 36
FEJA \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Equity Capital Structure, Percentage		50.00%
Public Utilities, Approved Return on Equity, Percentage		5.80%
Increase Decrease in Return on Equity for Energy Savings Goals		2.00%
FEJA 2018 through 2021 [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
FEJA Energy Efficiency Investments		$ 99
IEIMA \| 2017 IEIMA Revenue Requirement Reconciliation [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets		54
IEIMA \| 2016 IEIMA Revenue Requirement Reconciliation [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		24
IEIMA \| 2015 IEMA Revenue Requirement Reconciliation [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets			$ 68
IEIMA \| Final Rate Order \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 17
Subsequent Event [Member] \| Electric Distribution \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 50
Subsequent Event [Member] \| Natural Gas \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 16
Subsequent Event [Member] \| Pending Rate Case \| Natural Gas \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 49
Public Utilities, Requested Return on Equity, Percentage	10.30%
Rate Base	$ 1,600
Subsequent Event [Member] \| Components of Rate Increase \| Natural Gas \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 42