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- Definition

Incentive adder to FERC allowed base return on common equity

+ References

No definition available.

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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.

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- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2420-110228

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

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- Definition

Carrying amount as of the balance sheet date of investments in power and distribution projects that are expected to be realized after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
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- Definition

The amount of cash paid for the settlement of litigation or for other legal issues during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3536-108585

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- Definition

Percentage of public utility's approved return on equity.

+ References

No definition available.

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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.

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- Definition

The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Federal) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 128	$ 110
Provision for Callaway construction and operating license		0	0	$ 69
New Nuclear Energy Center COL [Member]
Public Utilities, General Disclosures [Line Items]
Provision for Callaway construction and operating license		69
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		92	78
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		19	12
Provision for Callaway construction and operating license		0	$ 0	$ 69
Union Electric Company \| New Nuclear Energy Center COL [Member]
Public Utilities, General Disclosures [Line Items]
Investments in Power and Distribution Projects		69
Provision for Callaway construction and operating license		$ 69
Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		12.38%
Payments for Legal Settlements		$ 21
Midwest Independent Transmission System Operator, Inc [Member] \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Payments for Legal Settlements		$ 17
Administrative Law Judge [Member] \| Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		9.70%
Incentive adder to FERC allowed base return on common equity		0.50%
Administrative Law Judge [Member] \| Midwest Independent Transmission System Operator, Inc [Member] \| Maximum
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		10.20%
Pending Ferc Case \| Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 42
Pending Ferc Case \| Midwest Independent Transmission System Operator, Inc [Member] \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 25
Final Rate Order \| Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		10.32%
Customer Requested Rate on Equity		9.15%
Incentive adder to FERC allowed base return on common equity		0.50%
Final Rate Order \| Midwest Independent Transmission System Operator, Inc [Member] \| Maximum
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		10.82%
Subsequent Event [Member] \| Midwest Independent Transmission System Operator, Inc [Member] \| Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 27
Subsequent Event [Member] \| Midwest Independent Transmission System Operator, Inc [Member] \| Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 23