Rate And Regulatory Matters	3 Months Ended
Mar. 31, 2017
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 March 2017 Electric Rate Order In March 2017, the MoPSC issued an order approving a unanimous stipulation and agreement in Ameren Missouri’s July 2016 regulatory rate review. The order resulted in a $3.4 billion revenue requirement, which is a $92 million increase in Ameren Missouri’s annual revenue requirement for electric service, compared to its prior revenue requirement established in the MoPSC's April 2015 electric rate order. The new rates, base level of expenses, and amortizations became effective on April 1, 2017. The order included the continued use of the FAC and the regulatory tracking mechanisms for pension and postretirement benefits, uncertain income tax positions, and renewable energy standards that the MoPSC authorized in earlier electric rate orders. These regulatory tracking mechanisms provide for a base level of expense to be reflected in Ameren Missouri’s base electric rates with differences in the actual expenses incurred recorded as a regulatory asset or liability. Excluding cost reductions associated with reduced sales volumes, the base level of net energy costs decrease by $54 million from the base level established in the MoPSC's April 2015 electric rate order. Changes in amortizations and the base level of expenses for the other regulatory tracking mechanisms, including extending the amortization period of certain regulatory assets, reduced expenses by $26 million from the base levels established in the MoPSC's April 2015 electric rate order. ATXI’s Mark Twain Project The Mark Twain project is a MISO-approved transmission line to be located in northeast Missouri. In April 2016, the MoPSC granted ATXI a certificate of convenience and necessity for the Mark Twain project conditioned upon ATXI obtaining county assents for road crossings. None of the five county commissions have approved ATXI’s requests for the assents. In October 2016, ATXI filed suit in the circuit courts for each of the five counties to obtain the assents for the original project route. A decision in each of the five lawsuits is expected in late 2017. In March 2017, the MoPSC’s April 2016 order was vacated by the Missouri Court of Appeals, Western District, which ruled that the MoPSC could not lawfully grant a certificate of convenience and necessity conditioned upon ATXI obtaining the assents. ATXI is evaluating whether to appeal the March 2017 Court of Appeals decision to the Missouri Supreme Court. In April 2017, ATXI reached agreements in principle with a cooperative electric company in northeast Missouri and with Ameren Missouri to locate portions of the Mark Twain project on existing transmission line corridors, resulting in a proposed alternative project route. ATXI will finalize the proposed alternative project route and then request assents for road crossings from the five affected counties. If all five county commissions provide assents for the proposed alternative project route, ATXI will then seek MoPSC approval. ATXI plans to complete the project in 2019; however, delays in obtaining the assents and approval from the MoPSC could delay completion. Illinois IEIMA & FEJA Under the provisions of Illinois law, Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity. This revenue requirement reconciliation qualifies as an alternative revenue program under GAAP. 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 and investment return. As of March 31, 2017, Ameren Illinois had recorded regulatory assets of $24 million and $54 million to reflect its 2016 revenue requirement reconciliation adjustment, which was included in the April 2017 formula rate update discussed below, and the approved 2015 revenue requirement reconciliation adjustment, with interest, respectively. For the three months ended March 31, 2017, Ameren Illinois recorded a regulatory asset of $46 million to reflect the difference between Ameren Illinois’ estimate of its revenue requirement and the revenue requirement reflected in customer rates. In April 2017, Ameren Illinois filed with the ICC its annual electric distribution service formula rate update to establish the revenue requirement used for 2018 rates. Pending ICC approval, this update filing will result in a $16 million decrease in Ameren Illinois’ electric distribution service revenue requirement beginning in January 2018. This update reflects an increase to the annual formula rate based on 2016 actual costs and expected net plant additions for 2017, an increase to include the 2016 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2015 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2017, consistent with the ICC’s December 2016 annual update filing order. The revenue requirement decrease in the April 2017 update filing is a result of the 2015 revenue requirement reconciliation adjustment being larger than the 2016 revenue requirement reconciliation adjustment. An ICC decision regarding the revenue requirement to be used for customer rates in 2018 is expected by December 2017. The FEJA revised certain portions of the IEIMA, including extending the IEIMA formula ratemaking process through 2022, and clarifying that a common equity ratio of up to, and including, 50% is prudent. Beginning in 2017, the FEJA provides that Ameren Illinois will recover, within the following two years, its electric distribution revenue requirement for a given year, independent of actual sales volumes. Prior to the FEJA, Ameren Illinois’ interim period revenue recognition was volume-based, as revenues were affected by the timing of sales volumes due to seasonal rates and changes in volumes resulting from, among other things, weather and energy efficiency. This previous revenue recognition method resulted in more revenues during the third quarter, and less revenues during the other quarters of each year. Beginning in 2017, in connection with the decoupling provisions of the FEJA, Ameren Illinois changed its method used to recognize interim period revenue. Ameren Illinois will now recognize revenue consistent with the timing of actual incurred electric distribution recoverable costs and recognize revenue associated with the expected return on its rate base ratably over the year. Ameren Illinois recognized $46 million and $11 million of electric distribution revenue to reflect the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates for the three months ended March 31, 2017 and March 31, 2016, respectively. 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 FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15%. In September 2016, the FERC issued a final order in the November 2013 complaint case, which lowered the allowed base return on common equity for the 15-month period of November 2013 to February 2015 to 10.32%, or a 10.82% total allowed return on common equity with the inclusion of the 50 basis point incentive adder for participation in an RTO. The order required customer refunds, with interest, to be issued for that 15-month period. During the first quarter of 2017, Ameren and Ameren Illinois refunded $21 million and $17 million, respectively, related to the November 2013 complaint case. In addition, the 10.82% allowed return on common equity has been reflected in rates since September 2016. As 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 further reduction in the allowed base return on common equity for the FERC-regulated transmission rate base under the MISO tariff. In June 2016, an administrative law judge issued an initial decision in the February 2015 complaint case, which if approved by the FERC, would lower the allowed base return on common equity for the 15-month period of February 2015 to May 2016 to 9.70%, or a 10.20% total allowed return on equity with the inclusion of the 50 basis point incentive adder for participation in an RTO and require customer refunds, with interest, for that 15-month period. The timing of the issuance of the final order in the February 2015 complaint case, which will prospectively replace the current 10.82% allowed return on equity, is uncertain for two reasons. First, at this time the FERC lacks a quorum of three commissioners. Second, the United States Court of Appeals for the District of Columbia recently vacated and remanded to the FERC an order in a separate case in which the FERC established the allowed base return on common equity methodology used in the two MISO complaint cases described above. Ameren is unable to predict the impact of the outcome of the United States Court of Appeals for the District of Columbia Circuit’s remand on the MISO FERC complaint cases at this time. As of March 31, 2017, Ameren and Ameren Illinois recorded current regulatory liabilities of $41 million and $24 million, respectively, to reflect the expected refunds, including interest, associated with the reduced allowed returns on common equity in the initial decision in the February 2015 complaint case. Ameren Missouri does not expect that a reduction in the FERC-allowed base return on common equity 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

March 2017 Electric Rate Order

In March 2017, the MoPSC issued an order approving a unanimous stipulation and agreement in Ameren Missouri’s July 2016 regulatory rate review. The order resulted in a $3.4 billion revenue requirement, which is a $92 million increase in Ameren Missouri’s annual revenue requirement for electric service, compared to its prior revenue requirement established in the MoPSC's April 2015 electric rate order. The new rates, base level of expenses, and amortizations became effective on April 1, 2017.

The order included the continued use of the FAC and the regulatory tracking mechanisms for pension and postretirement benefits, uncertain income tax positions, and renewable energy standards that the MoPSC authorized in earlier electric rate orders. These regulatory tracking mechanisms provide for a base level of expense to be reflected in Ameren Missouri’s base electric rates with differences in the actual expenses incurred recorded as a regulatory asset or liability. Excluding cost reductions associated with reduced sales volumes, the base level of net energy costs decrease by $54 million from the base level established in the MoPSC's April 2015 electric rate order. Changes in amortizations and the base level of expenses for the other regulatory tracking mechanisms, including extending the amortization period of certain regulatory assets, reduced expenses by $26 million from the base levels established in the MoPSC's April 2015 electric rate order.

ATXI’s Mark Twain Project

The Mark Twain project is a MISO-approved transmission line to be located in northeast Missouri. In April 2016, the MoPSC granted ATXI a certificate of convenience and necessity for the Mark Twain project conditioned upon ATXI obtaining county assents for road crossings. None of the five county commissions have approved ATXI’s requests for the assents. In October 2016, ATXI filed suit in the circuit courts for each of the five counties to obtain the assents for the original project route. A decision in each of the five lawsuits is expected in late 2017. In March 2017, the MoPSC’s April 2016 order was vacated by the Missouri Court of Appeals, Western District, which ruled that the MoPSC could not lawfully grant a certificate of convenience and necessity conditioned upon ATXI obtaining the assents. ATXI is evaluating whether to appeal the March 2017 Court of Appeals decision to the Missouri Supreme Court.

In April 2017, ATXI reached agreements in principle with a cooperative electric company in northeast Missouri and with Ameren Missouri to locate portions of the Mark Twain project on existing transmission line corridors, resulting in a proposed alternative project route. ATXI will finalize the proposed alternative project route and then request assents for road crossings from the five affected counties. If all five county commissions provide assents for the proposed alternative project route, ATXI will then seek MoPSC approval.

ATXI plans to complete the project in 2019; however, delays in obtaining the assents and approval from the MoPSC could delay completion.

Illinois

IEIMA & FEJA

Under the provisions of Illinois law, Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity. This revenue requirement reconciliation qualifies as an alternative revenue program under GAAP. 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 and investment return. As of March 31, 2017, Ameren Illinois had recorded regulatory assets of $24 million and $54 million to reflect its 2016 revenue requirement reconciliation adjustment, which was included in the April 2017 formula rate update discussed below, and the approved 2015 revenue requirement reconciliation adjustment, with interest, respectively. For the three months ended March 31, 2017, Ameren Illinois recorded a regulatory asset of $46 million to reflect the difference between Ameren Illinois’ estimate of its revenue requirement and the revenue requirement reflected in customer rates.

In April 2017, Ameren Illinois filed with the ICC its annual electric distribution service formula rate update to establish the revenue requirement used for 2018 rates. Pending ICC approval, this update filing will result in a $16 million decrease in Ameren Illinois’ electric distribution service revenue requirement beginning in January 2018. This update reflects an increase to the annual formula rate based on 2016 actual costs and expected net plant additions for 2017, an increase to include the 2016 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2015 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2017, consistent with the ICC’s December 2016 annual update filing order. The revenue requirement decrease in the April 2017 update filing is a result of the 2015 revenue requirement reconciliation adjustment being larger than the 2016 revenue requirement reconciliation adjustment. An ICC decision regarding the revenue requirement to be used for customer rates in 2018 is expected by December 2017.

The FEJA revised certain portions of the IEIMA, including extending the IEIMA formula ratemaking process through 2022, and clarifying that a common equity ratio of up to, and including, 50% is prudent. Beginning in 2017, the FEJA provides that Ameren Illinois will recover, within the following two years, its electric distribution revenue requirement for a given year, independent of actual sales volumes. Prior to the FEJA, Ameren Illinois’ interim period revenue recognition was volume-based, as revenues were affected by the timing of sales volumes due to seasonal rates and changes in volumes resulting from, among other things, weather and energy efficiency. This previous revenue recognition method resulted in more revenues during the third quarter, and less revenues during the other quarters of each year. Beginning in 2017, in connection with the decoupling provisions of the FEJA, Ameren Illinois changed its method used to recognize interim period revenue. Ameren Illinois will now recognize revenue consistent with the timing of actual incurred electric distribution recoverable costs and recognize revenue associated with the expected return on its rate base ratably over the year. Ameren Illinois recognized $46 million and $11 million of electric distribution revenue to reflect the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates for the three months ended March 31, 2017 and March 31, 2016, respectively.

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 FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15%. In September 2016, the FERC issued a final order in the November 2013 complaint case, which lowered the allowed base return on common equity for the 15-month period of November 2013 to February 2015 to 10.32%, or a 10.82% total allowed return on common equity with the inclusion of the 50 basis point incentive adder for participation in an RTO. The order required customer refunds, with interest, to be issued for that 15-month period. During the first quarter of 2017, Ameren and Ameren Illinois refunded $21 million and $17 million, respectively, related to the November 2013 complaint case. In addition, the 10.82% allowed return on common equity has been reflected in rates since September 2016.

As 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 further reduction in the allowed base return on common equity for the FERC-regulated transmission rate base under the MISO tariff. In June 2016, an administrative law judge issued an initial decision in the February 2015 complaint case, which if approved by the FERC, would lower the allowed base return on common equity for the 15-month period of February 2015 to May 2016 to 9.70%, or a 10.20% total allowed return on equity with the inclusion of the 50 basis point incentive adder for participation in an RTO and require customer refunds, with interest, for that 15-month period. The timing of the issuance of the final order in the February 2015 complaint case, which will prospectively replace the current 10.82% allowed return on equity, is uncertain for two reasons. First, at this time the FERC lacks a quorum of three commissioners. Second, the United States Court of Appeals for the District of Columbia recently vacated and remanded to the FERC an order in a separate case in which the FERC established the allowed base return on common equity methodology used in the two MISO complaint cases described above. Ameren is unable to predict the impact of the outcome of the United States Court of Appeals for the District of Columbia Circuit’s remand on the MISO FERC complaint cases at this time.

As of March 31, 2017, Ameren and Ameren Illinois recorded current regulatory liabilities of $41 million and $24 million, respectively, to reflect the expected refunds, including interest, associated with the reduced allowed returns on common equity in the initial decision in the February 2015 complaint case. Ameren Missouri does not expect that a reduction in the FERC-allowed base return on common equity would be material to its results of operations, financial position, or liquidity.