[Line Items] for Accounting Policies [Table]

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- Definition

Impact related to tax benefit associated with share-based compensation plan other than an employee stock ownership plan (ESOP). Includes, but is not limited to, excess tax benefit.

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Adjustments related to Income Tax Benefit from Share-Based payment per diluted share

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- Definition

Asbestos trust fund balance

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- Definition

Cash impact for realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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Noncash Or Part Noncash Capital Contribution From Parent

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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.

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- Definition

Percentage reporting unit exceeded carrying value

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- Definition

Proceeds from Asset Sale Held in Escrow

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Provision For Depreciation Percent Of Average Depreciable Cost

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- Definition

Public Utilities, Area Serviced

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Public Utilities, Estimated Population of Service Territory

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Public Utilities, Number of Customers

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- Definition

Sharing Level For Fac

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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.

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

Cash outflow to owners or shareholders, excluding ordinary dividends. Includes special dividends.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
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- Definition

Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

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

The cash inflow from parent as a source of financing that is recorded as additional paid in capital.

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Aggregate cash proceeds received from a combination of transactions in which noncurrent assets are sold, which may include the sale of a business, an investment in an affiliate (including an equity method investee), property, plant and equipment and intangible assets. Excludes sales of trading, available-for-sale, and held-to-maturity securities.

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

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-SubTopic 10
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Summary Of Significant Accounting Policies (Narrative) (Details) $ / shares in Units, people in Millions, customer in Millions, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2016 USD ($) mi² customer people $ / shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Accounting Policies [Line Items]
Goodwill			$ 411.0	$ 411.0
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities					$ 18.0
Adjustments related to Income Tax Benefit from Share-Based Payment			$ 21.0
Adjustments related to Income Tax Benefit from Share-Based payment per diluted share \| $ / shares			$ 0.09
Excess Tax Benefit from Share-Based Compensation				2.0
Payments Related to Tax Withholding for Share-based Compensation			$ 83.0	12.0	14.0
Percentage reporting unit exceeded carrying value			40.00%
Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced \| mi²			40,000
Public Utilities, Estimated Population of Service Territory \| people			3.1
Goodwill			$ 411.0	411.0
Capital contribution from parent			$ 0.0	25.0	15.0
Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced \| mi²			24,000
Public Utilities, Estimated Population of Service Territory \| people			2.8
Capital contribution from parent			$ 44.0	224.0	215.0
Noncash Or Part Noncash Capital Contribution From Parent			0.0	38.0	9.0
Return of capital to parent			$ 0.0	$ 0.0	(215.0)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities					$ 18.0
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				$ 53.0	$ 2.0
Elgin, Gibson City and Grand Tower Energy Centers [Member]
Accounting Policies [Line Items]
Proceeds from Sales of Business, Affiliate and Productive Assets	$ 168.0	$ 137.5
Proceeds from Asset Sale Held in Escrow				$ 14.0
Asbestos-Related
Accounting Policies [Line Items]
Asbestos trust fund balance			$ 22.0
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			$ 22.0
Ameren Illinois Electric Distribution [Member]
Accounting Policies [Line Items]
Goodwill			238.0
Ameren Illinois Gas [Member]
Accounting Policies [Line Items]
Goodwill			80.0
Ameren Illinois Transmission [Member]
Accounting Policies [Line Items]
Goodwill			93.0
Ameren Transmission [Member]
Accounting Policies [Line Items]
Goodwill			$ 93.0