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Summary Of Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2012
Accounting Policies [Abstract]  
Schedule Of Materials And Supplies
The following table presents a breakdown of materials and supplies for each of the Ameren Companies at December 31, 2012, and 2011:
 
Ameren(a)
 
Ameren Missouri
 
Ameren Illinois
2012
 
 
 
 
 
Fuel(b)
$
276

 
$
198

 
$
—

Gas stored underground
131

 
18

 
113

Other materials and supplies
297

 
181

 
60


$
704

 
$
397

 
$
173

2011
 
 
 
 
 
Fuel(b)
$
251

 
$
150

 
$
—

Gas stored underground
171

 
22

 
149

Other materials and supplies
290

 
176

 
50


$
712

 
$
348

 
$
199

(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
The following table presents the annual allowance for funds used during construction rates that were utilized during 2012, 2011 and 2010:
 
2012
 
2011
 
2010
Ameren
8% - 9%

 
8% - 9% 

 
8% - 9% 

Ameren Missouri
8
%
 
8
%
 
8
%
Ameren Illinois
9
%
 
9
%
 
9
%
Schedule Of Amortization Based On Usage Of Renewable Energy Credits And Emission Allowances
The following table does not include the intangible asset impairment charges referenced below.
 
2012
 
2011
 
2010
Ameren Missouri
$ (a)

 
$ (a)

 
$
6

Ameren Illinois
4

 
3

 
7

Other(b)(c)
3

 
3

 
22

Ameren(c)
$
7

 
$
6

 
$
35

(a)
Less than $1 million.
(b)
Consists of renewable energy credit expense for Marketing Company and emission allowance expense for Genco and AERG.
Schedule Of Excise Taxes
The following table presents excise taxes recorded in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” for the years ended 2012, 2011 and 2010:
 
2012
 
2011
 
2010
Ameren Missouri
$
139

 
$
137

 
$
130

Ameren Illinois
54

 
57

 
59

Ameren
$
193

 
$
194

 
$
189
Schedule Of Asset Retirement Obligations
The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the years 2012 and 2011:
 
Ameren
Missouri(a)
 
Ameren
Illinois(b)
 
Genco
 
AERG
 
Ameren(a)
 
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(e)
(54
)
 
(c)

 
(6
)
 
(6
)
 
(66
)
 
Balance at December 31, 2011
$
328

 
$
3

 
$
71

 
$
31

 
$
433

(f) 
Liabilities incurred
—

 
—

 
2

 
—

 
2

 
Liabilities settled
(1
)
 
(c)

 
(5
)
 
(c)

 
(6
)
 
Accretion in 2012(d)
18

 
(c)

 
4

 
2

 
24

 
Change in estimates(g)
1

 
(c)

 
(3
)
 
2

 
(c)

 
Balance at December 31, 2012
$
346

 
$
3

 
$
69

 
$
35

 
$
453

(h) 
(a)
The nuclear decommissioning trust fund assets of $408 million and $357 million as of December 31, 2012, and 2011, 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 changed its fair value estimate 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 their fair value estimates related to retirement costs for asbestos removal, river structures and their CCR storage facilities.
(f)
Balance included $5 million in "Other current liabilities" on the balance sheet as of December 31, 2011.
(g)
Ameren Missouri and Genco changed their fair value estimates for asbestos removal. The estimates for asbestos removal costs at Genco's Hutsonville and Meredosia energy centers decreased because less asbestos than anticipated was found in the energy centers' structures during reviews made after the closure of these energy centers, and because removal was more cost efficient than anticipated due to the closure. Additionally, Genco and AERG changed their fair value estimates related to updated retirement dates for certain CCR storage facilities.
(h)
Balance included $8 million in "Other current liabilities" on the balance sheet as of December 31, 2012.