Summary Of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2011

Summary Of Significant Accounting Policies [Abstract]


	Ameren^(a)		Ameren Missouri		Ameren Illinois		Genco
2011:
Fuel^(b)	$	251	$	150	$	-	$	76
Gas stored underground		171		22		149		-
Other materials and supplies		290		176		50		46
	$	712	$	348	$	199	$	122
2010:
Fuel^(b)	$	255	$	152	$	-	$	81
Gas stored underground		175		22		152		-
Other materials and supplies		277		167		46		49
	$	707	$	341	$	198	$	130

(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

(b)	Consists of coal, oil, paint, propane, and tire chips.

Schedule Of Rates Used For Allowance For Funds Used During Construction


	2011		2010		2009
Ameren		8% - 9%		8% - 9%		6% - 9%
Ameren Missouri		8		8		6
Ameren Illinois		9		9		9

Schedule Of Amortization Based On Usage Of Renewable Energy Credits And Emission Allowances


	2011			2010		2009
Ameren Missouri	$	(a	)	$	6	$	2
Ameren Illinois		3			7		9
Genco^(b)		2			18		24
Other^(b)(c)		1			4		5
Ameren^(b)	$	6		$	35	$	40

(a)	Less than $1 million.

(b)	Includes allowances consumed that were recorded through purchase accounting.

(c)	Consists of renewable energy credit expense for Marketing Company and emission allowances expense for AERG.

Schedule Of Excise Taxes


	2011		2010		2009
Ameren Missouri	$	137	$	130	$	112
Ameren Illinois		57		59		56
Ameren	$	194	$	189	$	168

Schedule Of Asset Retirement Obligations


	Ameren Missouri^(a)			Ameren Illinois^(b)			Genco			AERG			Ameren^(a)
Balance at December 31, 2009	$	331		$	5		$	65		$	33		$	434
Liabilities incurred		5			(c	)		3			-			8
Liabilities settled		(4	)		(c	)		(c	)		(c	)		(4	)
Accretion in 2010^(d)		19			1			4			2			26
Change in estimates^(e)		12			(3	)		2			(c	)		11
Balance at December 31, 2010	$	363		$	3		$	74		$	35		$	475
Liabilities incurred		-			-			(c	)		-			(c	)
Liabilities settled		(1	)		(c	)		(2	)		(c	)		(3	)
Accretion in 2011^(d)		20			(c	)		5			2			27
Change in estimates^(f)		(54	)		(c	)		(6	)		(6	)		(66	)
Balance at December 31, 2011	$	328		$	3		$	71	^(g)	$	31		$	433	^(g)

(a)	The nuclear decommissioning trust fund assets of $357 million and $337 million as of December 31, 2011, and 2010, respectively, were restricted for decommissioning of the Callaway energy center.

(b)	Balance included in "Other deferred credits and liabilities" on the balance sheet.

(c)	Less than $1 million.

(d)	Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.

(e)	Ameren Missouri and Genco changed their estimates for asbestos removal. Additionally, Genco changed the estimates related to retirement costs for its coal combustion byproduct storage areas.

(f)

Ameren Missouri changed estimates related to its Callaway energy center decommissioning costs because of a cost study performed in 2011 and a decline in the cost escalation factor assumptions. Additionally, Ameren Missouri, Genco and AERG changed estimates related to retirement costs for asbestos removal, river structures and their coal combustion byproduct storage areas.

(g)	Balance included $5 million in "Other current liabilities" on the balance sheet as of December 31, 2011.

- Definition

The entire disclosure for an asset retirement obligation and the associated long-lived asset. An asset retirement obligation is a legal obligation associated with the disposal or retirement from service of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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- Definition

Tabular disclosure of amortizable intangibles assets, in total and by major class, including the gross carrying amount and accumulated amortization. A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company.

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