Rate And Regulatory Matters [Line Items]

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- Definition

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

+ References

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- Definition

Transmission Line Miles

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- Definition

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

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- Definition

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

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

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- Definition

Percentage of public utility's requested return on equity.

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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

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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Rate And Regulatory Matters (Narrative-Illinois) (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) mi	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,578		$ 1,582
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	922		$ 883
Ameren Illinois Company \| Electric Distribution \| Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	15
Ameren Illinois Company \| Gas Distribution \| Pending Rate Case
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 45
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%
Rate Base	$ 1,200
Public Utilities, Requested Return on Equity, Percentage	9.60%
Ameren Transmission Company of Illinois \| Spoon River Project
Rate And Regulatory Matters [Line Items]
Transmission Line Miles \| mi	46
IEIMA \| Ameren Illinois Company \| Electric Distribution \| 2015 IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 52
Public Utilities, Requested Rate Increase (Decrease), Amount	109
IEIMA \| Ameren Illinois Company \| Electric Distribution \| 2014 IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Regulatory assets	103
IEIMA \| Ameren Illinois Company \| Electric Distribution \| 2013 IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 14
Icc Staff Recommendation \| IEIMA \| Ameren Illinois Company \| Electric Distribution \| 2015 IEMA Revenue Requirement Reconciliation \| Subsequent Event
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 107