Rate And Regulatory Matters	3 Months Ended
Mar. 31, 2014
Public Utilities, General Disclosures [Abstract]
RATE AND REGULATORY MATTERS	RATE AND REGULATORY MATTERS Below is a summary of updates to significant regulatory proceedings and related lawsuits. See also Note 2 - Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K. We are unable to predict the ultimate outcome of these matters, the timing of the final decisions of the various agencies and courts, or the impact on our results of operations, financial position, or liquidity. Missouri Accounting Authority Order In July 2011, Ameren Missouri filed a request with the MoPSC for an accounting authority order that would allow Ameren Missouri to defer fixed costs totaling $36 million that were not previously recovered from Noranda as a result of the loss of load caused by the severe 2009 ice storm for potential recovery in a future electric rate case. In November 2013, the MoPSC issued an order approving Ameren Missouri’s request for an accounting authority order, which will allow Ameren Missouri to seek recovery of these fixed costs in a future electric rate case. Ameren Missouri will seek to recover these fixed costs in its next electric rate case, which it will file by July 15, 2014. In February 2014, MIEC and MoOPC filed appeals of the accounting authority order to the Missouri Court of Appeals, Western District. Earnings Complaint and Rate Shift Complaint Cases In February 2014, Ameren Missouri’s largest customer, Noranda, and 37 residential customers filed an earnings complaint case and a rate shift complaint case with the MoPSC. In the earnings complaint case, Noranda and the residential customers asserted that Ameren Missouri’s electric service business is earning more than the 9.8% return on equity authorized in the MoPSC's December 2012 electric rate order and requested that the MoPSC approve a $67 million reduction to Ameren Missouri’s annual revenue requirement. Included in Noranda’s request is a reduction of Ameren Missouri’s authorized return on equity to 9.4%. The rate shift complaint case seeks to reduce Noranda’s electricity cost with an offsetting increase in electricity cost for Ameren Missouri’s other customers. While the rate shift proposal is revenue neutral to Ameren Missouri, Ameren Missouri does not believe that the proposed reduction to Noranda’s electric rates, which are significantly below Ameren Missouri’s cost of service, is appropriate or in the best interests of Ameren Missouri’s other electric customers. The MoOPC and MIEC have intervened in the earnings complaint case. While the MoPSC has no time requirement by which it must issue an order in these cases, it has adopted procedural schedules that would render a decision in the rate shift case by August 6, 2014, and in the earnings complaint case by September 26, 2014. Ameren Missouri does not believe that a reduction in electric service rates is justified and will file testimony that supports that position, which is consistent with Ameren Missouri’s expected July 2014 rate case filing. The MoPSC has 11 months from when Ameren Missouri files its request to issue an order. Illinois IEIMA Under the provisions of the IEIMA, Ameren Illinois’ electric delivery service rates are subject to an annual revenue requirement reconciliation to its actual costs. Throughout each year, Ameren Illinois records a regulatory asset or a regulatory liability and a corresponding increase or decrease to operating revenues for any differences between the revenue requirement in effect for that year and its estimate of the probable increase or decrease in the revenue requirement expected to ultimately be approved by the ICC based on that year's actual costs incurred. As of March 31, 2014, Ameren Illinois had recorded a regulatory asset of $24 million to reflect its expected 2014 revenue requirement reconciliation adjustment, with interest. In September 2012 and December 2012, the ICC issued orders in Ameren Illinois’ IEIMA performance-based formula rate filings. Ameren Illinois appealed both orders to the Appellate Court of the Fourth District of Illinois. The primary issues Ameren Illinois appealed were the rate treatment of accumulated deferred income taxes and vacation obligations as well as the calculation of Ameren Illinois’ capital structure. In December 2013, the appellate court rendered its decision upholding the ICC’s September and December 2012 orders. Ameren Illinois filed an appeal to the Illinois Supreme Court in March 2014. In December 2013, the ICC issued an order in Ameren Illinois' annual update filing, which was based on 2012 recoverable costs and expected net plant additions for 2013. The ICC order established rates for 2014. In February 2014, Ameren Illinois filed an appeal to the Appellate Court of the Fourth District of Illinois regarding the calculation of its capital structure and the rate treatment of accumulated deferred income taxes related to the transfer of former Ameren Missouri assets in Illinois to Ameren Illinois. In April 2014, Ameren Illinois filed with the ICC its annual electric delivery service formula rate update to establish the revenue requirement used to set rates for 2015. Pending ICC approval, Ameren Illinois’ update filing will result in a $206 million increase in Ameren Illinois’ electric delivery service revenue requirement beginning in January 2015. This update reflects an increase to the annual formula rate based on 2013 actual costs and expected net plant additions for 2014; an increase to include the annual reconciliation of the revenue requirement in effect for 2013 to the actual costs incurred in that year; and an increase resulting from the conclusion of a refund to customers in 2014 for the 2012 revenue requirement reconciliation. An ICC decision on this April 2014 filing is expected by December 2014. 2013 Natural Gas Delivery Service Rate Case In December 2013, the ICC issued a rate order that approved an increase in revenues for natural gas delivery service of $32 million. The revenue increase was based on a 9.1% return on equity, a capital structure composed of 51.7% common equity, and a rate base of $1.1 billion. The rate order was based on a 2014 future test year. The rate changes became effective January 1, 2014. In March 2014, Ameren Illinois filed an appeal of the allowed return on equity included in the ICC's order with the Appellate Court of the Fourth District of Illinois. Ameren Illinois sought a 10.4% return on equity in this rate case. Federal 2011 Wholesale Distribution Rate Case In January 2011, Ameren Illinois filed a request with FERC to increase its annual revenues for electric delivery service for its wholesale customers. These wholesale distribution revenues are treated as a deduction from Ameren Illinois’ revenue requirement in retail rate filings with the ICC. In March 2011, FERC issued an order authorizing the proposed rates to take effect, subject to refund when the final rates are determined. Ameren Illinois has reached a settlement with four of its nine wholesale customers, which have been approved by FERC and refunds have been issued. The impasse with the remaining five wholesale customers is awaiting final FERC action. In November 2012, a FERC administrative law judge issued an initial decision, which is now pending before FERC. The timing of a decision from FERC is uncertain and subsequent appeals are possible. In accordance with the administrative law judge's initial decision, Ameren and Ameren Illinois have both included on their balance sheets in “Current regulatory liabilities” an estimate of $14 million and $13 million as of March 31, 2014, and December 31, 2013, respectively, for the refund due to the remaining wholesale customers relating to billings for the period from March 2011 through March 2014. Ameren Illinois Electric Transmission Rate Refund In July 2012, FERC issued an order concluding that Ameren Illinois improperly included acquisition premiums, primarily goodwill, in determining the common equity used in its electric transmission formula rate, and thereby inappropriately recovered a higher amount from its electric transmission customers. The order required Ameren Illinois to make refunds to customers for such improperly included amounts. In August 2012, Ameren Illinois filed a request for a rehearing of this order. It is unknown when FERC will rule on Ameren's rehearing request, as it is under no deadline to do so. Ameren Illinois submitted a refund report in November 2012 and concluded that no refund was warranted. Several wholesale customers filed a protest with FERC regarding Ameren's conclusion that no refund was warranted. In June 2013, FERC issued an order that rejected Ameren Illinois' November 2012 refund report and provided guidance as to the filing of a new refund report. In July 2013, Ameren Illinois filed a revised refund report based on the guidance provided in the June 2013 order, as well as a request for a rehearing of that order. Ameren Illinois' July 2013 refund report also concluded that no refund was warranted. Ameren Illinois estimated the maximum pretax charge to earnings for this possible refund obligation through December 31, 2014 would be $19 million, before interest charges. If Ameren Illinois were to determine that a refund to its electric transmission customers is probable, a charge to earnings would be recorded for the refund in the period in which that determination was made. FERC Complaint Case In November 2013, a customer group filed a complaint case with FERC seeking a reduction in the allowed return on common equity to 9.15%, as well as a limit on the common equity ratio, under the MISO tariff. Currently, the FERC-allowed return on common equity for MISO transmission owners is 12.38%. This complaint case could result in a reduction to Ameren Illinois' and ATXI's allowed return on common equity. That reduction could also result in a refund for transmission service revenues earned after the filing of the complaint case in November 2013. FERC has not issued an order in this case, and it is under no deadline to do so. Ameren is not able to predict if or how FERC will rule on this complaint case. Ameren Missouri Power Purchase Agreement with Entergy Beginning in 2005, FERC issued a series of orders addressing a complaint filed in 2001 by the Louisiana Public Service Commission against Entergy and certain of its affiliates. The complaint alleged unjust and unreasonable cost allocations. As a result of the FERC orders, Entergy began billing Ameren Missouri in 2007 for additional charges under a 165-megawatt power purchase agreement, which expired August 31, 2009. In May 2012, FERC issued an order stating that Entergy should not have included additional charges to Ameren Missouri under the power purchase agreement. Pursuant to the order, in June 2012, Entergy paid Ameren Missouri $31 million. In July 2012, Entergy filed an appeal of FERC's January 2010 and May 2012 orders to the United States Court of Appeals for the District of Columbia Circuit, which was subsequently dismissed on a procedural issue. In November 2013, Entergy refiled the appeal of FERC's May 2012 order with the United States Court of Appeals for the District of Columbia Circuit. Ameren is not able to predict when or how the court will rule on Entergy's appeal. The Louisiana Public Service Commission appealed FERC’s orders regarding Louisiana Public Service Commission’s complaint against Entergy Services, Inc. to the United States Court of Appeals for the District of Columbia Circuit. In April 2008, that court ordered further FERC proceedings regarding Louisiana Public Service Commission’s complaint. The court ordered FERC to explain its previous denial of retroactive refunds and the implementation of prospective charges. FERC’s decision on remand of the retroactive impact of these issues could have a financial impact on Ameren Missouri. Ameren Missouri is unable to predict when or how FERC will respond to the court’s decisions. Ameren Missouri estimates that it could incur an additional expense of up to $25 million if FERC orders retroactive application for the years 2001 to 2005. Ameren Missouri believes that the likelihood of incurring any expense is not probable, and therefore no liability has been recorded as of March 31, 2014. Combined Construction and Operating License In 2008, Ameren Missouri filed an application with the NRC for a COL for a new nuclear unit at Ameren Missouri's existing Callaway County, Missouri, energy center site. In 2009, Ameren Missouri suspended its efforts to build a new nuclear unit at its existing Missouri nuclear energy center site, and the NRC suspended review of the COL application. Ameren Missouri estimated the total cost required to obtain a small modular reactor COL to be $80 million to $100 million. As of March 31, 2014, Ameren Missouri had capitalized investments of $69 million for the development of a new nuclear energy center. Ameren Missouri is currently evaluating all potential nuclear technologies in order to maintain an option for nuclear power in the future. All of Ameren Missouri's capitalized investments for the development of a new nuclear energy center will remain capitalized while management pursues options to maximize the value of its investment. If efforts to license additional nuclear generation are abandoned or management concludes it is probable the costs incurred will be disallowed in rates, a charge to earnings would be recognized in the period in which that determination is made.

RATE AND REGULATORY MATTERS

Below is a summary of updates to significant regulatory proceedings and related lawsuits. See also Note 2 - Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K. We are unable to predict the ultimate outcome of these matters, the timing of the final decisions of the various agencies and courts, or the impact on our results of operations, financial position, or liquidity.

Missouri

Accounting Authority Order

In July 2011, Ameren Missouri filed a request with the MoPSC for an accounting authority order that would allow Ameren Missouri to defer fixed costs totaling $36 million that were not previously recovered from Noranda as a result of the loss of load caused by the severe 2009 ice storm for potential recovery in a future electric rate case. In November 2013, the MoPSC issued an order approving Ameren Missouri’s request for an accounting authority order, which will allow Ameren Missouri to seek recovery of these fixed costs in a future electric rate case. Ameren Missouri will seek to recover these fixed costs in its next electric rate case, which it will file by July 15, 2014. In February 2014, MIEC and MoOPC filed appeals of the accounting authority order to the Missouri Court of Appeals, Western District.

Earnings Complaint and Rate Shift Complaint Cases

In February 2014, Ameren Missouri’s largest customer, Noranda, and 37 residential customers filed an earnings complaint case and a rate shift complaint case with the MoPSC. In the earnings complaint case, Noranda and the residential customers asserted that Ameren Missouri’s electric service business is earning more than the 9.8% return on equity authorized in the MoPSC's December 2012 electric rate order and requested that the MoPSC approve a $67 million reduction to Ameren Missouri’s annual revenue requirement. Included in Noranda’s request is a reduction of Ameren Missouri’s authorized return on equity to 9.4%. The rate shift complaint case seeks to reduce Noranda’s electricity cost with an offsetting increase in electricity cost for Ameren Missouri’s other customers. While the rate shift proposal is revenue neutral to Ameren Missouri, Ameren Missouri does not believe that the proposed reduction to Noranda’s electric rates, which are significantly below Ameren Missouri’s cost of service, is appropriate or in the best interests of Ameren Missouri’s other electric customers. The MoOPC and MIEC have intervened in the earnings complaint case.

While the MoPSC has no time requirement by which it must issue an order in these cases, it has adopted procedural schedules that would render a decision in the rate shift case by August 6, 2014, and in the earnings complaint case by September 26, 2014. Ameren Missouri does not believe that a reduction in electric service rates is justified and will file testimony that supports that position, which is consistent with Ameren Missouri’s expected July 2014 rate case filing. The MoPSC has 11 months from when Ameren Missouri files its request to issue an order.

Illinois

IEIMA

Under the provisions of the IEIMA, Ameren Illinois’ electric delivery service rates are subject to an annual revenue requirement reconciliation to its actual costs. Throughout each year, Ameren Illinois records a regulatory asset or a regulatory liability and a corresponding increase or decrease to operating revenues for any differences between the revenue requirement in effect for that year and its estimate of the probable increase or decrease in the revenue requirement expected to ultimately be approved by the ICC based on that year's actual costs incurred. As of March 31, 2014, Ameren Illinois had recorded a regulatory asset of $24 million to reflect its expected 2014 revenue requirement reconciliation adjustment, with interest.

In September 2012 and December 2012, the ICC issued orders in Ameren Illinois’ IEIMA performance-based formula rate filings. Ameren Illinois appealed both orders to the Appellate Court of the Fourth District of Illinois. The primary issues Ameren Illinois appealed were the rate treatment of accumulated deferred income taxes and vacation obligations as well as the calculation of Ameren Illinois’ capital structure. In December 2013, the appellate court rendered its decision upholding the ICC’s September and December 2012 orders. Ameren Illinois filed an appeal to the Illinois Supreme Court in March 2014.

In December 2013, the ICC issued an order in Ameren Illinois' annual update filing, which was based on 2012 recoverable costs and expected net plant additions for 2013. The ICC order established rates for 2014. In February 2014, Ameren Illinois filed an appeal to the Appellate Court of the Fourth District of Illinois regarding the calculation of its capital structure and the rate treatment of accumulated deferred income taxes related to the transfer of former Ameren Missouri assets in Illinois to Ameren Illinois.

In April 2014, Ameren Illinois filed with the ICC its annual electric delivery service formula rate update to establish the revenue requirement used to set rates for 2015. Pending ICC approval, Ameren Illinois’ update filing will result in a $206 million increase in Ameren Illinois’ electric delivery service revenue requirement beginning in January 2015. This update reflects an increase to the annual formula rate based on 2013 actual costs and expected net plant additions for 2014; an increase to include the annual reconciliation of the revenue requirement in effect for 2013 to the actual costs incurred in that year; and an increase resulting from the conclusion of a refund to customers in 2014 for the 2012 revenue requirement reconciliation. An ICC decision on this April 2014 filing is expected by December 2014.

2013 Natural Gas Delivery Service Rate Case

In December 2013, the ICC issued a rate order that approved an increase in revenues for natural gas delivery service of $32 million. The revenue increase was based on a 9.1% return on equity, a capital structure composed of 51.7% common equity, and a rate base of $1.1 billion. The rate order was based on a 2014 future test year. The rate changes became effective January 1, 2014. In March 2014, Ameren Illinois filed an appeal of the allowed return on equity included in the ICC's order with the Appellate Court of the Fourth District of Illinois. Ameren Illinois sought a 10.4% return on equity in this rate case.

Federal

2011 Wholesale Distribution Rate Case

In January 2011, Ameren Illinois filed a request with FERC to increase its annual revenues for electric delivery service for its wholesale customers. These wholesale distribution revenues are treated as a deduction from Ameren Illinois’ revenue requirement in retail rate filings with the ICC. In March 2011, FERC issued an order authorizing the proposed rates to take effect, subject to refund when the final rates are determined. Ameren Illinois has reached a settlement with four of its nine wholesale customers, which have been approved by FERC and refunds have been issued. The impasse with the remaining five wholesale customers is awaiting final FERC action. In November 2012, a FERC administrative law judge issued an initial decision, which is now pending before FERC. The timing of a decision from FERC is uncertain and subsequent appeals are possible. In accordance with the administrative law judge's initial decision, Ameren and Ameren Illinois have both included on their balance sheets in “Current regulatory liabilities” an estimate of $14 million and $13 million as of March 31, 2014, and December 31, 2013, respectively, for the refund due to the remaining wholesale customers relating to billings for the period from March 2011 through March 2014.

Ameren Illinois Electric Transmission Rate Refund

In July 2012, FERC issued an order concluding that Ameren Illinois improperly included acquisition premiums, primarily goodwill, in determining the common equity used in its electric transmission formula rate, and thereby inappropriately recovered a higher amount from its electric transmission customers. The order required Ameren Illinois to make refunds to customers for such improperly included amounts. In August 2012, Ameren Illinois filed a request for a rehearing of this order. It is unknown when FERC will rule on Ameren's rehearing request, as it is under no deadline to do so.

Ameren Illinois submitted a refund report in November 2012 and concluded that no refund was warranted. Several wholesale customers filed a protest with FERC regarding Ameren's conclusion that no refund was warranted. In June 2013, FERC issued an order that rejected Ameren Illinois' November 2012 refund report and provided guidance as to the filing of a new refund report. In July 2013, Ameren Illinois filed a revised refund report based on the guidance provided in the June 2013 order, as well as a request for a rehearing of that order. Ameren Illinois' July 2013 refund report also concluded that no refund was warranted. Ameren Illinois estimated the maximum pretax charge to earnings for this possible refund obligation through December 31, 2014 would be $19 million, before interest charges. If Ameren Illinois were to determine that a refund to its electric transmission customers is probable, a charge to earnings would be recorded for the refund in the period in which that determination was made.

FERC Complaint Case

In November 2013, a customer group filed a complaint case with FERC seeking a reduction in the allowed return on common equity to 9.15%, as well as a limit on the common equity ratio, under the MISO tariff. Currently, the FERC-allowed return on common equity for MISO transmission owners is 12.38%. This complaint case could result in a reduction to Ameren Illinois' and ATXI's allowed return on common equity. That reduction could also result in a refund for transmission service revenues earned after the filing of the complaint case in November 2013. FERC has not issued an order in this case, and it is under no deadline to do so. Ameren is not able to predict if or how FERC will rule on this complaint case.

Ameren Missouri Power Purchase Agreement with Entergy

Beginning in 2005, FERC issued a series of orders addressing a complaint filed in 2001 by the Louisiana Public Service Commission against Entergy and certain of its affiliates. The complaint alleged unjust and unreasonable cost allocations. As a result of the FERC orders, Entergy began billing Ameren Missouri in 2007 for additional charges under a 165-megawatt power purchase agreement, which expired August 31, 2009. In May 2012, FERC issued an order stating that Entergy should not have included additional charges to Ameren Missouri under the power purchase agreement. Pursuant to the order, in June 2012, Entergy paid Ameren Missouri $31 million. In July 2012, Entergy filed an appeal of FERC's January 2010 and May 2012 orders to the United States Court of Appeals for the District of Columbia Circuit, which was subsequently dismissed on a procedural issue. In November 2013, Entergy refiled the appeal of FERC's May 2012 order with the United States Court of Appeals for the District of Columbia Circuit. Ameren is not able to predict when or how the court will rule on Entergy's appeal.

The Louisiana Public Service Commission appealed FERC’s orders regarding Louisiana Public Service Commission’s complaint against Entergy Services, Inc. to the United States Court of Appeals for the District of Columbia Circuit. In April 2008, that court ordered further FERC proceedings regarding Louisiana Public Service Commission’s complaint. The court ordered FERC to explain its previous denial of retroactive refunds and the implementation of prospective charges. FERC’s decision on remand of the retroactive impact of these issues could have a financial impact on Ameren Missouri. Ameren Missouri is unable to predict when or how FERC will respond to the court’s decisions. Ameren Missouri estimates that it could incur an additional expense of up to $25 million if FERC orders retroactive application for the years 2001 to 2005. Ameren Missouri believes that the likelihood of incurring any expense is not probable, and therefore no liability has been recorded as of March 31, 2014.

Combined Construction and Operating License

In 2008, Ameren Missouri filed an application with the NRC for a COL for a new nuclear unit at Ameren Missouri's existing Callaway County, Missouri, energy center site. In 2009, Ameren Missouri suspended its efforts to build a new nuclear unit at its existing Missouri nuclear energy center site, and the NRC suspended review of the COL application.

Ameren Missouri estimated the total cost required to obtain a small modular reactor COL to be $80 million to $100 million. As of March 31, 2014, Ameren Missouri had capitalized investments of $69 million for the development of a new nuclear energy center. Ameren Missouri is currently evaluating all potential nuclear technologies in order to maintain an option for nuclear power in the future.

All of Ameren Missouri's capitalized investments for the development of a new nuclear energy center will remain capitalized while management pursues options to maximize the value of its investment. If efforts to license additional nuclear generation are abandoned or management concludes it is probable the costs incurred will be disallowed in rates, a charge to earnings would be recognized in the period in which that determination is made.