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Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accounting Policies [Line Items]
Excise tax expense	$ 55	$ 45
Union Electric Company [Member]
Accounting Policies [Line Items]
Excise tax expense	33	27
Ameren Illinois Company [Member]
Accounting Policies [Line Items]
Excise tax expense	$ 22	$ 18

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.

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