Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Accounting Policies [Line Items]
Excise tax expense	$ 57us-gaap_ExciseAndSalesTaxes	$ 60us-gaap_ExciseAndSalesTaxes
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	34	34
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 23	$ 26

The amount of excise and sales taxes included in sales and revenues, which are then deducted as a cost of sales. Includes excise taxes, which are applied to specific types of transactions or items (such as gasoline or alcohol); and sales, use and value added taxes, which are applied to a broad class of revenue-producing transactions involving a wide range of goods and services.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 45

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6408196&loc=d3e61097-111654

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration