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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

Percentage of public utility's approved return on equity.

+ 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.

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

+ References

No definition available.

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- Definition

Percentage of public utility's requested return on equity.

+ References

No definition available.

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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) (USD $)	3 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended
	Mar. 31, 2015	Jan. 31, 2015	Dec. 31, 2014	Apr. 30, 2015
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,577,000,000us-gaap_RegulatoryAssetsNoncurrent		1,582,000,000us-gaap_RegulatoryAssetsNoncurrent
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	883,000,000		883,000,000
Ameren Illinois Company \| Electric Distribution [Member] \| Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	15,000,000
Ameren Illinois Company \| Gas Distribution [Member] \| Pending Rate Case [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		53,000,000
Public Utilities, Requested Return on Equity, Percentage		10.25%
Public Utilities, Requested Equity Capital Structure, Percentage		50.00%
Rate Base		1,200,000,000
Ameren Illinois Company \| Gas Distribution [Member] \| Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage			9.10%
Ameren Illinois Company \| Gas Distribution [Member] \| Rate order appeal [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage			10.40%
IEIMA [Member] \| Ameren Illinois Company \| Electric Distribution [Member] \| 2015 IEMA Revenue Requirement Reconciliation [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	8,000,000
IEIMA [Member] \| Ameren Illinois Company \| Electric Distribution [Member] \| 2015 IEMA Revenue Requirement Reconciliation [Member] \| Subsequent Event [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				110,000,000
IEIMA [Member] \| Ameren Illinois Company \| Electric Distribution [Member] \| 2014 IEMA Revenue Requirement Reconciliation [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	101,000,000
IEIMA [Member] \| Ameren Illinois Company \| Electric Distribution [Member] \| 2013 IEMA Revenue Requirement Reconciliation [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 52,000,000