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Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Accounting Policies [Line Items]
Excise tax expense	$ 48	$ 44	$ 93	$ 95
Ameren Missouri [Member]
Accounting Policies [Line Items]
Excise tax expense	38	34	65	63
Ameren Illinois Company [Member]
Accounting Policies [Line Items]
Excise tax expense	$ 10	$ 10	$ 28	$ 32

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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