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Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accounting Policies [Line Items]
Excise tax expense	$ 59	$ 55	$ 163	$ 148
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	49	46	120	111
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 10	$ 9	$ 43	$ 37

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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