Closing Common Share Price

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- Definition

As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

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- Definition

The weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, using a decimal to express in number of years.

+ References

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- Definition

The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

+ References

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- Definition

The difference between the sale price or salvage price and the book value of a property, plant, and equipment asset that was sold or retired during the reporting period. This element refers to the gain (loss).

+ References

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-Name Accounting Standards Codification
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-SubTopic 10
-Section 45
-Paragraph 28
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- Definition

The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Subparagraph b
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

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- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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- Definition

The cash inflow from sale of machinery and equipment.

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-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
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-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

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- Definition

Equity-based compensation cost during the period with respect to the award, which will be recognized in income (as well as the total recognized tax benefit) or capitalized as part of the cost of an asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

+ Details

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- Definition

The gross amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name FASB Interpretation (FIN)
-Number 48
-Paragraph 21
-Subparagraph a
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Unrecognized Tax Benefit
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-Name Accounting Standards Codification
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- Definition

The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name FASB Interpretation (FIN)
-Number 48
-Paragraph 21
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Summary of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended			1 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Merchant Generation [Member]	Jun. 30, 2011 Ameren Missouri [Member]	Jun. 30, 2011 Ameren Energy Generating Company [Member]	Jun. 30, 2011 Ameren Energy Generating Company [Member]	Dec. 31, 2010 Ameren Energy Generating Company [Member]	Jun. 30, 2011 Renewable Energy Credits [Member]	Jan. 31, 2011 Performance Share Units [Member]
Fair value of share unit												$ 31.41
Closing common share price					$ 28.19
Three-year risk-free rate			1.08%
Volatility rate, minimum			22.00%
Volatility rate, maximum			36.00%
Share-based compensation expense	$ 4	$ 2	$ 7	$ 7
Employee service share-based compensation, tax benefit from compensation expense	2	1	3	3
Unrecognized share-based compensation expense	27		27
Unrecognized compensation costs on nonvested awards, weighted average period of recognition, in months			22
Pretax impairment charge						2	1		1
Book value	4		4		7					3	3
Unrecognized tax benefits	198		198
Unrecognized tax benefits that would impact effective tax rate	2		2
Percentage of EEI not owned by Ameren	20.00%		20.00%
Proceeds from sale of machinery and equipment								45
Pretax gain recognized on sale of machinery and equipment								$ 8