Asset Impairments	6 Months Ended
Asset Impairments	Jun. 30, 2012
Asset Impairments	NOTE 11 - ASSET IMPAIRMENTS We evaluate long-lived assets classified as held and used for impairment when events or changes in circumstances indicate that the carrying value, or book value, of such assets may not be recoverable. Under applicable accounting guidance, whether an impairment has occurred is determined by comparing the estimated undiscounted cash flows attributable to the assets with the carrying value of the assets. If the carrying value exceeds the estimated undiscounted cash flows, we recognize an impairment charge equal to the carrying value of the assets in excess of estimated fair value. Power prices in the Midwest affect the amount of revenues and cash flows Merchant Generation and Genco can realize by marketing power into the wholesale and retail markets. During the first quarter of 2012, the observable market price for power for delivery in the current year and in future years in the Midwest sharply declined below 2011 levels primarily because of declining natural gas prices and the impact of the stay of the CSAPR. For example, from December 31, 2011, through February 29, 2012, the market price for power at the Indiana Hub for delivery in the current year decreased by 14%. As a result of this sharp decline in the market price of power and the related impact on electric margins, Genco decelerated the construction of two scrubbers at its Newton energy center in February 2012. The sharp decline in the market price of power in the first quarter of 2012 and the related impact on electric margins, as well as the deceleration of construction of Genco’s Newton energy center scrubber project, caused Merchant Generation and Genco to evaluate, during the first quarter of 2012, whether the carrying values of their coal-fired energy centers were recoverable. The carrying values of Merchant Generation’s and Genco’s energy centers exceeded their estimated fair values. However, under the applicable accounting guidance, if undiscounted future cash flows from these long-lived assets exceed their carrying values, the assets are deemed unimpaired, and no impairment loss is recognized, even if the carrying values of the assets exceed estimated fair values. Only AERG’s Duck Creek energy center’s carrying value exceeded its estimated undiscounted future cash flows. As a result, Ameren recorded a noncash pretax asset impairment charge of $628 million to reduce the carrying value of AERG’s Duck Creek energy center to its estimated fair value during the first quarter of 2012. This impairment charge was included in Ameren’s results and in the Merchant Generation’s segment results for the first quarter of 2012 and the six months ended June 30,2012. Key assumptions used in the determination of estimated undiscounted cash flows of the Merchant Generation and Genco long-lived assets tested for impairment included the forward price projections for energy and fuel costs, the expected life of the energy center, environmental compliance costs and strategies, and operating costs. Those same cash flow assumptions, along with a discount rate, were used to estimate the fair value of the long-lived assets of the Duck Creek energy center. The fair value estimate of the long-lived assets of the Duck Creek energy center was based on the income approach, which considers discounted future cash flows. The fair value estimate was determined using observable inputs and significant unobservable inputs, which are Level 3 inputs as defined by accounting guidance for fair value measurements. After the impairment of the Duck Creek energy center, Merchant Generation and Genco believed the carrying value of their energy centers exceeded their estimated fair values by an amount significantly in excess of $1 billion. Merchant Generation and Genco will continue to monitor the market price for power and the related impact on electric margin and other events or changes in circumstances that indicate that the carrying value of their energy centers may not be recoverable as compared to their undiscounted cash flows. Merchant Generation and Genco could recognize additional, material long-lived asset impairment charges in the future as a result of factors outside their control, such as changes in power or fuel costs, administrative action or inaction by regulatory agencies and new environmental laws and regulations that could reduce the expected useful lives of Merchant Generation’s and Genco’s energy centers, and also as a result of factors that may be within their control, such as a failure to achieve forecasted operating results and cash flows, unfavorable changes in forecasted operating results and cash flows, or decisions to shut down, mothball or sell their energy centers. The Duck Creek energy center asset impairment charge did not result in a violation of any Ameren debt covenants or counterparty agreements. See Note 1 - Summary of Significant Accounting Policies for information regarding the intangible asset impairment recorded during the second quarter of 2011.
Ameren Energy Generating Company [Member]
Asset Impairments	NOTE 11 - ASSET IMPAIRMENTS We evaluate long-lived assets classified as held and used for impairment when events or changes in circumstances indicate that the carrying value, or book value, of such assets may not be recoverable. Under applicable accounting guidance, whether an impairment has occurred is determined by comparing the estimated undiscounted cash flows attributable to the assets with the carrying value of the assets. If the carrying value exceeds the estimated undiscounted cash flows, we recognize an impairment charge equal to the carrying value of the assets in excess of estimated fair value. Power prices in the Midwest affect the amount of revenues and cash flows Merchant Generation and Genco can realize by marketing power into the wholesale and retail markets. During the first quarter of 2012, the observable market price for power for delivery in the current year and in future years in the Midwest sharply declined below 2011 levels primarily because of declining natural gas prices and the impact of the stay of the CSAPR. For example, from December 31, 2011, through February 29, 2012, the market price for power at the Indiana Hub for delivery in the current year decreased by 14%. As a result of this sharp decline in the market price of power and the related impact on electric margins, Genco decelerated the construction of two scrubbers at its Newton energy center in February 2012. The sharp decline in the market price of power in the first quarter of 2012 and the related impact on electric margins, as well as the deceleration of construction of Genco’s Newton energy center scrubber project, caused Merchant Generation and Genco to evaluate, during the first quarter of 2012, whether the carrying values of their coal-fired energy centers were recoverable. The carrying values of Merchant Generation’s and Genco’s energy centers exceeded their estimated fair values. However, under the applicable accounting guidance, if undiscounted future cash flows from these long-lived assets exceed their carrying values, the assets are deemed unimpaired, and no impairment loss is recognized, even if the carrying values of the assets exceed estimated fair values. Only AERG’s Duck Creek energy center’s carrying value exceeded its estimated undiscounted future cash flows. As a result, Ameren recorded a noncash pretax asset impairment charge of $628 million to reduce the carrying value of AERG’s Duck Creek energy center to its estimated fair value during the first quarter of 2012. This impairment charge was included in Ameren’s results and in the Merchant Generation’s segment results for the first quarter of 2012 and the six months ended June 30,2012. Key assumptions used in the determination of estimated undiscounted cash flows of the Merchant Generation and Genco long-lived assets tested for impairment included the forward price projections for energy and fuel costs, the expected life of the energy center, environmental compliance costs and strategies, and operating costs. Those same cash flow assumptions, along with a discount rate, were used to estimate the fair value of the long-lived assets of the Duck Creek energy center. The fair value estimate of the long-lived assets of the Duck Creek energy center was based on the income approach, which considers discounted future cash flows. The fair value estimate was determined using observable inputs and significant unobservable inputs, which are Level 3 inputs as defined by accounting guidance for fair value measurements. After the impairment of the Duck Creek energy center, Merchant Generation and Genco believed the carrying value of their energy centers exceeded their estimated fair values by an amount significantly in excess of $1 billion. Merchant Generation and Genco will continue to monitor the market price for power and the related impact on electric margin and other events or changes in circumstances that indicate that the carrying value of their energy centers may not be recoverable as compared to their undiscounted cash flows. Merchant Generation and Genco could recognize additional, material long-lived asset impairment charges in the future as a result of factors outside their control, such as changes in power or fuel costs, administrative action or inaction by regulatory agencies and new environmental laws and regulations that could reduce the expected useful lives of Merchant Generation’s and Genco’s energy centers, and also as a result of factors that may be within their control, such as a failure to achieve forecasted operating results and cash flows, unfavorable changes in forecasted operating results and cash flows, or decisions to shut down, mothball or sell their energy centers. The Duck Creek energy center asset impairment charge did not result in a violation of any Ameren debt covenants or counterparty agreements. See Note 1 - Summary of Significant Accounting Policies for information regarding the intangible asset impairment recorded during the second quarter of 2011.