Rate And Regulatory Matters	3 Months Ended
Mar. 31, 2016
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 MEEIA 2013 The MEEIA 2013 performance incentive allowed Ameren Missouri an opportunity to earn additional revenues by achieving certain customer energy efficiency goals, including $19 million if 100% of the goals were achieved during the three-year period, with the potential to earn a larger performance incentive if Ameren Missouri’s energy savings exceeded those goals. In November 2015, the MoPSC issued an order that clarified how an input used in the calculation of the performance incentive would be determined. Ameren Missouri filed an appeal of the order with the Missouri Court of Appeals, Western District. If the Missouri Court of Appeals upholds the MoPSC order, the MEEIA 2013 performance incentive will be significantly less than the performance incentive calculated using Ameren Missouri’s interpretation. A decision from the Missouri Court of Appeals is expected in 2016. Separately, an order from the MoPSC determining the MEEIA 2013 performance incentive is also expected in 2016. Ameren Missouri has not recorded any revenues associated with the MEEIA 2013 performance incentive. Ameren Missouri believes it will ultimately be found to have exceeded 100% of the customer energy efficiency goals, and it therefore expects to recognize revenues relating to the MEEIA 2013 performance incentive of at least $19 million in 2016. Noranda Ameren Missouri supplies electricity to Noranda’s aluminum smelter located in southeast Missouri under a long-term power supply agreement. In the first quarter of 2016, Noranda idled production at its aluminum smelter. In addition, Noranda filed voluntary petitions for a court-supervised restructuring process under Chapter 11 of the United States Bankruptcy Code. Noranda stated it would maintain the flexibility to restart operations at the smelter should conditions allow. Ameren Missouri has been working with Noranda, legislators, and other stakeholders on a potential legislative solution to support Noranda’s operations. For utility service through March 31, 2016, Noranda prepaid an amount to Ameren Missouri in excess of its utility service usage. Ameren Missouri expects to be paid in full for utility services provided to Noranda. In its April 2015 electric rate order, the MoPSC approved a rate design that established $78 million in annual revenues, net of fuel and purchased power costs, as Noranda’s portion of Ameren Missouri’s revenue requirement. The portion of Ameren Missouri’s annual revenue requirement reflected in Noranda’s electric rate is based on the smelter using approximately 4.2 million megawatthours annually, which is almost 100% of its operating capacity. Ameren Missouri’s rates, including those for Noranda, are seasonal. Noranda’s summer base rate (June through September) is $45.78 per megawatthour, and its winter base rate (October through May) is $31.11 per megawatthour. In 2016, actual sales volumes to Noranda will be significantly below the sales volumes reflected in rates. As a result, Ameren Missouri will not fully recover its revenue requirement until rates are adjusted by the MoPSC in a future electric rate case to accurately reflect Noranda’s actual sales volumes. As a result of Noranda’s idled production described above, Ameren Missouri is applying a provision in its FAC tariff that, under certain circumstances, allows Ameren Missouri to retain a portion of the revenues from any off-system sales it makes as a result of reduced tariff sales to Noranda. The current market price of electricity is less than Noranda’s electric rate, and Ameren Missouri expects market prices to remain below Noranda’s electric rate during 2016. Accordingly, this FAC-tariff provision will not enable Ameren Missouri to fully recover its revenue requirement under current market conditions. Ameren Missouri expects to file an electric rate case in 2016 to reflect additional infrastructure investments and rising costs, including depreciation, transmission service, and property tax expenses, and expects the resulting new rates to reflect Noranda’s actual sales volumes, which would prospectively eliminate the impact of the current revenue shortfall from Noranda sales levels. The rate case would take place over a period of up to 11 months from the date of filing. Ameren Missouri may seek recovery of lost revenues in a filing with the MoPSC to recover certain costs incurred but not contemporaneously recovered by rate revenues as a result of Noranda's reduced operations. Ameren Missouri will continue to monitor Noranda’s sales volumes and to evaluate regulatory and legislative options that might mitigate adverse financial impacts. The reduction in Noranda’s sales volumes have adversely affected and will continue to adversely affect Ameren’s and Ameren Missouri’s results of operations, financial condition, and liquidity until customer rates are adjusted in a future rate case. ATXI Transmission Projects The Mark Twain project is a MISO-approved 95-mile transmission line located in northeast Missouri. In April 2016, the MoPSC granted ATXI a certificate of convenience and necessity for the Mark Twain project. Starting construction under the certificate is subject to ATXI obtaining assents from the five counties where the line will be constructed. The Mark Twain project is expected to be completed in 2018. Extended difficulties in obtaining the assents could delay the completion date. Illinois IEIMA Under the provisions of the IEIMA's performance-based formula rate-making framework, which currently extends through 2019, Ameren Illinois’ electric delivery service rates are subject to an annual revenue requirement reconciliation to its actual recoverable 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 reflected in customer rates 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 recoverable costs incurred. As of March 31, 2016, Ameren Illinois had recorded regulatory assets of $9 million, $65 million, and $82 million, to reflect its expected 2016 and 2015 revenue requirement reconciliation adjustments and the approved 2014 revenue requirement reconciliation adjustment, with interest, respectively. In April 2016, Ameren Illinois filed with the ICC its annual electric delivery service formula rate update to establish the revenue requirement used for 2017 rates. Pending ICC approval, Ameren Illinois’ update filing will result in a $14 million decrease in Ameren Illinois’ electric delivery service revenue requirement, beginning in January 2017. This update reflects an increase to the annual formula rate based on 2015 actual costs and expected net plant additions for 2016, an increase to include the 2015 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2014 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2016, consistent with the ICC’s December 2015 annual update filing order. As of December 31, 2015, Ameren Illinois had recorded a regulatory asset of $103 million related to the approved 2014 revenue requirement reconciliation adjustment. Federal FERC Complaint Cases In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for the FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15%. In December 2015, an administrative law judge issued an initial decision in the November 2013 complaint case that would lower the allowed base return on common equity to 10.32% and would require customer refunds to be issued for the 15-month period ending February 2015. The allowed base return on common equity in the initial decision was based on observable market data for the six months ended June 30, 2015. The FERC is expected to issue a final order on the November 2013 complaint case in the fourth quarter of 2016. Because the maximum FERC-allowed refund period for the November 2013 complaint case ended in February 2015, another customer complaint case was filed in February 2015. The February 2015 complaint case seeks a reduction in the allowed base return on common equity for the FERC-regulated transmission rate base under the MISO tariff to 8.67%. The initial decision from an administrative law judge in the February 2015 complaint case, which will subsequently require FERC approval, is expected to be issued in the second quarter of 2016. On January 6, 2015, a FERC-approved incentive adder of up to 50 basis points on the allowed base return on common equity for our participation in an RTO became effective. Beginning with its January 6, 2015 effective date, the incentive adder will reduce any refund to customers relating to a reduction of the allowed base return on common equity from the complaint cases discussed above. As of March 31, 2016, Ameren and Ameren Illinois had current regulatory liabilities of $55 million and $37 million, respectively, representing their estimates of the potential refunds from the November 12, 2013 refund effective date through March 31, 2016. Ameren and Ameren Illinois recorded liabilities to reflect the allowed base return on common equity in the initial decision for the November 2013 complaint case refund period and the observable market data for the six months ended December 31, 2015, for the February 2015 complaint case refund period. Ameren’s and Ameren Illinois’ liabilities also reflect the January 6, 2015 incentive adder discussed above. Ameren Missouri did not record a liability as of March 31, 2016, and it does not expect that a reduction in the FERC-allowed base return on common equity for MISO transmission owners would be material to its results of operations, financial position, or liquidity.

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

MEEIA 2013

The MEEIA 2013 performance incentive allowed Ameren Missouri an opportunity to earn additional revenues by achieving certain customer energy efficiency goals, including $19 million if 100% of the goals were achieved during the three-year period, with the potential to earn a larger performance incentive if Ameren Missouri’s energy savings exceeded those goals. In November 2015, the MoPSC issued an order that clarified how an input used in the calculation of the performance incentive would be determined. Ameren Missouri filed an appeal of the order with the Missouri Court of Appeals, Western District. If the Missouri Court of Appeals upholds the MoPSC order, the MEEIA 2013 performance incentive will be significantly less than the performance incentive calculated using Ameren Missouri’s interpretation. A decision from the Missouri Court of Appeals is expected in 2016. Separately, an order from the MoPSC determining the MEEIA 2013 performance incentive is also expected in 2016. Ameren Missouri has not recorded any revenues associated with the MEEIA 2013 performance incentive. Ameren Missouri believes it will ultimately be found to have exceeded 100% of the customer energy efficiency goals, and it therefore expects to recognize revenues relating to the MEEIA 2013 performance incentive of at least $19 million in 2016.

Noranda

Ameren Missouri supplies electricity to Noranda’s aluminum smelter located in southeast Missouri under a long-term power supply agreement.

In the first quarter of 2016, Noranda idled production at its aluminum smelter. In addition, Noranda filed voluntary petitions for a court-supervised restructuring process under Chapter 11 of the United States Bankruptcy Code. Noranda stated it would maintain the flexibility to restart operations at the smelter should conditions allow. Ameren Missouri has been working with Noranda, legislators, and other stakeholders on a potential legislative solution to support Noranda’s operations. For utility service through March 31, 2016, Noranda prepaid an amount to Ameren Missouri in excess of its utility service usage. Ameren Missouri expects to be paid in full for utility services provided to Noranda.

In its April 2015 electric rate order, the MoPSC approved a rate design that established $78 million in annual revenues, net of fuel and purchased power costs, as Noranda’s portion of Ameren Missouri’s revenue requirement. The portion of Ameren Missouri’s annual revenue requirement reflected in Noranda’s electric rate is based on the smelter using approximately 4.2 million megawatthours annually, which is almost 100% of its operating capacity. Ameren Missouri’s rates, including those for Noranda, are seasonal. Noranda’s summer base rate (June through September) is $45.78 per megawatthour, and its winter base rate (October through May) is $31.11 per megawatthour.

In 2016, actual sales volumes to Noranda will be significantly below the sales volumes reflected in rates. As a result, Ameren Missouri will not fully recover its revenue requirement until rates are adjusted by the MoPSC in a future electric rate case to accurately reflect Noranda’s actual sales volumes. As a result of Noranda’s idled production described above, Ameren Missouri is applying a provision in its FAC tariff that, under certain circumstances, allows Ameren Missouri to retain a portion of the revenues from any off-system sales it makes as a result of reduced tariff sales to Noranda. The current market price of electricity is less than Noranda’s electric rate, and Ameren Missouri expects market prices to remain below Noranda’s electric rate during 2016. Accordingly, this FAC-tariff provision will not enable Ameren Missouri to fully recover its revenue requirement under current market conditions.

Ameren Missouri expects to file an electric rate case in 2016 to reflect additional infrastructure investments and rising costs, including depreciation, transmission service, and property tax expenses, and expects the resulting new rates to reflect Noranda’s actual sales volumes, which would prospectively eliminate the impact of the current revenue shortfall from Noranda sales levels. The rate case would take place over a period of up to 11 months from the date of filing. Ameren Missouri may seek recovery of lost revenues in a filing with the MoPSC to recover certain costs incurred but not contemporaneously recovered by rate revenues as a result of Noranda's reduced operations. Ameren Missouri will continue to monitor Noranda’s sales volumes and to evaluate regulatory and legislative options that might mitigate adverse financial impacts. The reduction in Noranda’s sales volumes have adversely affected and will continue to adversely affect Ameren’s and Ameren Missouri’s results of operations, financial condition, and liquidity until customer rates are adjusted in a future rate case.

ATXI Transmission Projects

The Mark Twain project is a MISO-approved 95-mile transmission line located in northeast Missouri. In April 2016, the MoPSC granted ATXI a certificate of convenience and necessity for the Mark Twain project. Starting construction under the certificate is subject to ATXI obtaining assents from the five counties where the line will be constructed. The Mark Twain project is expected to be completed in 2018. Extended difficulties in obtaining the assents could delay the completion date.

Illinois

IEIMA

Under the provisions of the IEIMA's performance-based formula rate-making framework, which currently extends through 2019, Ameren Illinois’ electric delivery service rates are subject to an annual revenue requirement reconciliation to its actual recoverable 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 reflected in customer rates 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 recoverable costs incurred. As of March 31, 2016, Ameren Illinois had recorded regulatory assets of $9 million, $65 million, and $82 million, to reflect its expected 2016 and 2015 revenue requirement reconciliation adjustments and the approved 2014 revenue requirement reconciliation adjustment, with interest, respectively.

In April 2016, Ameren Illinois filed with the ICC its annual electric delivery service formula rate update to establish the revenue requirement used for 2017 rates. Pending ICC approval, Ameren Illinois’ update filing will result in a $14 million decrease in Ameren Illinois’ electric delivery service revenue requirement, beginning in January 2017. This update reflects an increase to the annual formula rate based on 2015 actual costs and expected net plant additions for 2016, an increase to include the 2015 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2014 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2016, consistent with the ICC’s December 2015 annual update filing order. As of December 31, 2015, Ameren Illinois had recorded a regulatory asset of $103 million related to the approved 2014 revenue requirement reconciliation adjustment.

Federal

FERC Complaint Cases

In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for the FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15%. In December 2015, an administrative law judge issued an initial decision in the November 2013 complaint case that would lower the allowed base return on common equity to 10.32% and would require customer refunds to be issued for the 15-month period ending February 2015. The allowed base return on common equity in the initial decision was based on observable market data for the six months ended June 30, 2015. The FERC is expected to issue a final order on the November 2013 complaint case in the fourth quarter of 2016.

Because the maximum FERC-allowed refund period for the November 2013 complaint case ended in February 2015, another customer complaint case was filed in February 2015. The February 2015 complaint case seeks a reduction in the allowed base return on common equity for the FERC-regulated transmission rate base under the MISO tariff to 8.67%. The initial decision from an administrative law judge in the February 2015 complaint case, which will subsequently require FERC approval, is expected to be issued in the second quarter of 2016.

On January 6, 2015, a FERC-approved incentive adder of up to 50 basis points on the allowed base return on common equity for our participation in an RTO became effective. Beginning with its January 6, 2015 effective date, the incentive adder will reduce any refund to customers relating to a reduction of the allowed base return on common equity from the complaint cases discussed above.

As of March 31, 2016, Ameren and Ameren Illinois had current regulatory liabilities of $55 million and $37 million, respectively, representing their estimates of the potential refunds from the November 12, 2013 refund effective date through March 31, 2016. Ameren and Ameren Illinois recorded liabilities to reflect the allowed base return on common equity in the initial decision for the November 2013 complaint case refund period and the observable market data for the six months ended December 31, 2015, for the February 2015 complaint case refund period. Ameren’s and Ameren Illinois’ liabilities also reflect the January 6, 2015 incentive adder discussed above. Ameren Missouri did not record a liability as of March 31, 2016, and it does not expect that a reduction in the FERC-allowed base return on common equity for MISO transmission owners would be material to its results of operations, financial position, or liquidity.