Rate And Regulatory Matters	6 Months Ended
Jun. 30, 2018
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 Missouri Senate Bill 564 On June 1, 2018, Missouri Senate Bill 564 was enacted. The section of the law applicable to the TCJA became effective immediately; the remaining sections, including the ability to elect PISA, become effective August 28, 2018. The law resulted in certain changes to Missouri utility laws that affect the regulation of Ameren Missouri’s electric service business. These changes include the reduction of customer rates to pass through the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA and, at each electric utility's election, the use of PISA. Electric utilities that do not elect to use PISA will be eligible to request permission to implement revenue decoupling of residential and other non-demand metered customer classes. The law required the MoPSC to authorize a reduction in Ameren Missouri’s rates to pass through the effect of the TCJA within 90 days of the law’s effective date. In July 2018, the MoPSC authorized Ameren Missouri to reduce its annual revenue requirement by $167 million and reflect that reduction in rates beginning August 1, 2018. The reduction included $74 million for the amortization of excess accumulated deferred income taxes. In addition, Ameren Missouri recorded a reduction to revenue and a corresponding regulatory liability of $47 million for the excess amounts collected in rates related to the TCJA from January 1, 2018, through June 30, 2018. An additional amount will be recorded for July 2018 revenues. The regulatory liability will be reflected in customer rates over a period of time to be determined by the MoPSC in the next regulatory rate review. Upon Ameren Missouri’s expected PISA election, it would be permitted to defer and recover 85% of the depreciation expense and return on rate base on certain property, plant, and equipment placed in-service after August 28, 2018, and not included in base rates. Eligible PISA deferrals would exclude amounts related to new coal-fired, nuclear, and natural gas generating units and service to new customer premises. Upon approval in a regulatory rate review, PISA deferrals would be added to rate base prospectively and earn a return based on Ameren Missouri’s weighted-average cost of capital over a recovery period of 20 years. For electric utilities electing to use PISA, additional provisions apply, including limitations on customer rate increases. Ameren Missouri’s customer rate increases would be limited to a 2.85% compound annual growth rate in the average overall customer rate per kilowatthour, applied to electric rates effective April 1, 2017, less half of the 2018 savings from the TCJA that was passed on to customers. Upon election to use PISA, Ameren Missouri’s electric base rates would be frozen until April 1, 2020. Recoveries under the MEEIA, the FAC, and the RESRAM riders would not be frozen; however, except for costs recoverable under the MEEIA rider, Ameren Missouri would be unable to recover any amounts above the 2.85% cap from customers. If rate changes from the FAC or the RESRAM riders would cause rates to temporarily exceed the 2.85% cap, the overage would be deferred for future recovery in the next regulatory rate review; however, rates established in such regulatory rate review would be subject to the rate cap. Any deferred overages approved for recovery would be subject to deferral and recovered in a manner consistent with costs recovered under PISA. Both the rate cap and PISA election would be effective through December 2023, unless Ameren Missouri requests and receives MoPSC approval of an extension through December 2028. Ameren Missouri’s expected PISA election will support Ameren Missouri's ability to invest approximately $1 billion of incremental capital over the 2019 to 2023 period to strengthen and modernize Missouri's electric grid. MoPSC Federal Income Tax Proceedings In February 2018, the MoPSC initiated proceedings to investigate how the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA should be reflected in rates paid by customers of Missouri’s regulated utilities, including rates paid by electric and natural gas customers of Ameren Missouri. The proceeding for Ameren Missouri’s electric service business was dismissed after Missouri Senate Bill 564 was enacted on June 1, 2018, but the proceeding is still pending for Ameren Missouri’s natural gas distribution business. As of June 30, 2018, the potential reduction in natural gas customer rates is immaterial. The MoPSC is under no deadline to issue an order in the natural gas proceeding. Wind Generation Facility and RESRAM In the second quarter of 2018, Ameren Missouri entered into an agreement with a subsidiary of Terra-Gen, LLC to acquire a 400-megawatt wind generation facility after construction. The facility is expected to be located in northeastern Missouri and to be completed in 2020. The acquisition is subject to certain conditions, including the issuance of a certificate of convenience and necessity by the MoPSC, obtaining a MISO transmission interconnection agreement, and approval by the FERC. Ameren Missouri has filed for the certificate of convenience and necessity with the MoPSC. This facility would help Ameren Missouri to comply with the state renewable energy standard. In addition, Ameren Missouri requested the MoPSC to authorize a proposed RESRAM that would allow Ameren Missouri to adjust customer rates, including recovery of interest at a short-term borrowing rate, on an annual basis without a traditional regulatory rate review. The RESRAM is designed to mitigate the impacts of regulatory lag for investments in wind generation and other renewables by providing more timely recovery of costs and would provide Ameren Missouri a greater opportunity to earn its allowed return on investment. Ameren Missouri anticipates a decision by January 2019 related to the certificate of convenience and necessity and proposed RESRAM. Renewable Choice Program In June 2018, the MoPSC approved Ameren Missouri’s Renewable Choice Program, which allows large commercial and industrial customers and municipalities to receive up to 100 percent of their energy from renewable resources. The tariff-based program is designed to recover the costs of the election, net of changes in the market price of such energy. Based on customer contracts, the program enables Ameren Missouri to supply up to 400 megawatts of renewable wind energy generation, up to 200 megawatts of which it could own. As applicable, the addition of generation by Ameren Missouri would be subject to the issuance of a certificate of convenience and necessity by the MoPSC, obtaining transmission interconnection agreements with the MISO or other RTOs, and approval by the FERC. This generation would be incremental to the expected renewable generation included in the 2017 IRP. Without extension, the option to elect into the program will terminate in the third quarter of 2023. MEEIA In June 2018, Ameren Missouri filed a proposed customer energy-efficiency plan with the MoPSC under the MEEIA. This filing proposed a six-year plan, which includes a portfolio of customer energy-efficiency programs, along with a cost recovery mechanism. If the plan is approved, beginning in March 2019, Ameren Missouri intends to invest an average of $92 million per program year in the proposed customer energy-efficiency programs. Ameren Missouri requested continued use of a MEEIA rider, which allows Ameren Missouri to collect from or refund to customers any difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs and its lost revenues. In addition, Ameren Missouri requested incentives to earn additional revenues by achieving certain customer energy-efficiency goals, increasing from $10 million to $24 million annually, for a total of $115 million over the six-year period if 100% of its annual customer energy-efficiency goals are achieved. A decision by the MoPSC in this proceeding is anticipated by the first quarter of 2019. The MEEIA 2016 program provided Ameren Missouri with a performance incentive to earn additional revenues by achieving certain customer energy-efficiency goals, including $27 million if 100% of the goals were achieved during the three-year period beginning March 2016, with the potential to earn more if Ameren Missouri’s energy savings exceeded those goals. In September 2017, Ameren Missouri received an order from the MoPSC approving Ameren Missouri’s energy savings results for the first year of the MEEIA 2016 programs. As a result of this order and in accordance with revenue recognition guidance, Ameren Missouri recognized $5 million of revenues in the first quarter of 2018 relating to the MEEIA 2016 performance incentive. In July 2018, the Missouri Supreme Court overturned a 2016 decision by the Missouri Court of Appeals, Western District, which had upheld a 2015 MoPSC order regarding the determination of a certain input used to calculate the MEEIA 2013 performance incentive, and remanded the matter to the MoPSC. The MoPSC is required to issue a revised order consistent with the Missouri Supreme Court’s ruling; however, there is no deadline to issue such order. Upon issuance of the order, Ameren Missouri expects to recognize an additional $9 million MEEIA 2013 performance incentive. Illinois Electric Distribution Service Rates In April 2018, Ameren Illinois filed its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2019 rates with the ICC. In July 2018, the ICC staff submitted its calculation of the revenue requirement included in Ameren Illinois’ update filing, recommending an amount comparable to Ameren Illinois’ filing. Pending ICC approval, this update filing will result in a $72 million increase in Ameren Illinois’ electric distribution service rates beginning in January 2019. This update reflects an increase to the annual formula rate based on 2017 actual costs and expected net plant additions for 2018 and an increase to include the 2017 revenue requirement reconciliation adjustment. It also includes a decrease for the conclusion of the 2016 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2018, consistent with the ICC’s December 2017 annual update filing order. An ICC decision in this proceeding is expected by December 2018. As of June 30, 2018, Ameren Illinois had recorded a regulatory asset of $62 million to reflect the difference between Ameren Illinois’ estimate of its 2018 revenue requirement and the revenue requirement reflected in customer rates, including interest. Electric Customer Energy-Efficiency Investments In June 2018, Ameren Illinois filed its annual electric customer energy-efficiency formula rate update to establish the revenue requirement to be used for 2019 rates with the ICC. Pending ICC approval, this update filing will result in 2019 rates for electric customer energy-efficiency investments of $34 million, which represents an increase of $20 million from the 2018 rates. An ICC decision regarding the revenue requirement to be used for customer rates in 2019 is expected by December 2018. Income Tax Regulatory Mechanisms In February 2018, the ICC granted Ameren Illinois’ request, filed in January 2018, to establish a rider to reduce Ameren Illinois’ electric distribution customer rates for the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA and the return of excess deferred taxes, net of the increase in state income taxes enacted in July 2017. Ameren Illinois' electric distribution customer rates were reduced as a result of the rider beginning in the first quarter of 2018. The estimated reduction of $50 million per year will continue through 2019, as base rates will reflect the current income tax rates starting in 2020. In April 2018, the ICC approved a rider for the difference between revenues billed under natural gas rates established pursuant to Ameren Illinois’ most recent natural gas rate order, and the revenues that would have been billed had the state and federal tax rate changes discussed above been in effect. The rider required Ameren Illinois to record this regulatory liability beginning January 25, 2018. Ameren Illinois’ natural gas customer rates were reduced as a result of the rider beginning in May 2018, with an estimated reduction of up to $17 million, substantially over a one-year period. 2018 Natural Gas Delivery Service Regulatory Rate Review In January 2018, Ameren Illinois filed a request with the ICC seeking approval to increase its annual rates for natural gas delivery service. In July 2018, Ameren Illinois and the ICC staff filed a stipulation and agreement with the ICC that, pending ICC approval, would result in an annual natural gas rate increase of $37 million, based on the terms of the agreement and subject to adjustments for updated rate case and other postretirement benefit expenses. This increase in annual rates includes a 9.87% return on common equity, a capital structure composed of 50% common equity, and a rate base of $1.6 billion. It also reflects the reduction in the federal corporate income tax rate as a result of the TCJA, as well as the increase in the Illinois corporate income tax rate that became effective in July 2017, which decreased the annual rates by approximately $17 million. In an attempt to reduce regulatory lag, Ameren Illinois used a 2019 future test year in this proceeding. A decision by the ICC in this proceeding is required by December 2018, with new rates expected to be effective in January 2019. Ameren Illinois cannot predict the level of any delivery service rate changes the ICC may approve, nor whether any rate changes that may eventually be approved will be sufficient to enable Ameren Illinois to recover its costs and to earn a reasonable return on investments when the rate changes go into effect. 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 2016, the FERC issued a final order in the November 2013 complaint case, which lowered the allowed base return on common equity to 10.32%, or a 10.82% total allowed return on common equity with the inclusion of a 50 basis point incentive adder for participation in an RTO, effective since September 2016. The 10.82% allowed return on common equity may be replaced prospectively after the FERC issues a final order in the February 2015 complaint case, discussed below. Since 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. MISO transmission owners subsequently filed a motion to dismiss the February 2015 complaint, as discussed below. The February 2015 complaint case seeks a further reduction in the allowed base return on common equity for 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. If approved by the FERC, it 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 a 50 basis point incentive adder for participation in an RTO. It would also require customer refunds, with interest, for that 15-month period. A final FERC order would also establish the allowed return on common equity that will apply prospectively from the effective date of such order, replacing the current 10.82% total return on common equity. In the second quarter of 2017, the United States Court of Appeals for the District of Columbia Circuit 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 the FERC is under no deadline to issue a final order, the timing of the issuance of the final order in the February 2015 complaint case is uncertain. In September 2017, MISO transmission owners, including Ameren Missouri, Ameren Illinois, and ATXI, filed a motion to dismiss the February 2015 complaint case with the FERC. The MISO transmission owners maintain that the February 2015 complaint was predicated on the now superseded 12.38% allowed base return on common equity and is therefore inapplicable given the current 10.32% allowed base return on common equity. The MISO transmission owners further maintain that the current 10.32% allowed base return on common equity has not been proven to be unjust and unreasonable based on information provided, including the base return on common equity methodology ranges set forth in the February 2015 complaint case and in the initial decision issued by an administrative law judge in June 2016. Additionally, the MISO transmission owners maintain that the February 2015 complaint should be dismissed because the approach utilized in the case to assert that a return on common equity was unjust and unreasonable was insufficient. That same approach was rejected by the United States Court of Appeals for the District of Columbia Circuit, as discussed above. The FERC is under no deadline to issue an order on this motion. As of June 30, 2018, Ameren and Ameren Illinois had recorded current regulatory liabilities of $43 million and $25 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. FERC Federal Income Tax Proceeding and Formula Rate Change In March 2018, the FERC granted a request filed in February 2018 by MISO transmission owners with forward-looking rate formulas, including Ameren Illinois and ATXI, to allow revisions to their 2018 electric transmission rates to reflect the effect of the reduction in federal income taxes enacted under the TCJA. Ameren Illinois and ATXI’s 2018 electric transmission rates have been reduced by $27 million and $23 million, respectively. In May 2018, the FERC accepted Ameren Illinois and ATXI tariff filings to change the formula rate calculation. The change allows for the recovery or refund of both excess deferred income taxes resulting from tax law or rate changes and effect of permanent income tax differences and will be reflected in Ameren Illinois and ATXI’s electric transmission rates starting in January 2019.

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

Missouri Senate Bill 564

On June 1, 2018, Missouri Senate Bill 564 was enacted. The section of the law applicable to the TCJA became effective immediately; the remaining sections, including the ability to elect PISA, become effective August 28, 2018. The law resulted in certain changes to Missouri utility laws that affect the regulation of Ameren Missouri’s electric service business. These changes include the reduction of customer rates to pass through the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA and, at each electric utility's election, the use of PISA. Electric utilities that do not elect to use PISA will be eligible to request permission to implement revenue decoupling of residential and other non-demand metered customer classes. The law required the MoPSC to authorize a reduction in Ameren Missouri’s rates to pass through the effect of the TCJA within 90 days of the law’s effective date. In July 2018, the MoPSC authorized Ameren Missouri to reduce its annual revenue requirement by $167 million and reflect that reduction in rates beginning August 1, 2018. The reduction included $74 million for the amortization of excess accumulated deferred income taxes. In addition, Ameren Missouri recorded a reduction to revenue and a corresponding regulatory liability of $47 million for the excess amounts collected in rates related to the TCJA from January 1, 2018, through June 30, 2018. An additional amount will be recorded for July 2018 revenues. The regulatory liability will be reflected in customer rates over a period of time to be determined by the MoPSC in the next regulatory rate review.

Upon Ameren Missouri’s expected PISA election, it would be permitted to defer and recover 85% of the depreciation expense and return on rate base on certain property, plant, and equipment placed in-service after August 28, 2018, and not included in base rates. Eligible PISA deferrals would exclude amounts related to new coal-fired, nuclear, and natural gas generating units and service to new customer premises. Upon approval in a regulatory rate review, PISA deferrals would be added to rate base prospectively and earn a return based on Ameren Missouri’s weighted-average cost of capital over a recovery period of 20 years. For electric utilities electing to use PISA, additional provisions apply, including limitations on customer rate increases. Ameren Missouri’s customer rate increases would be limited to a 2.85% compound annual growth rate in the average overall customer rate per kilowatthour, applied to electric rates effective April 1, 2017, less half of the 2018 savings from the TCJA that was passed on to customers. Upon election to use PISA, Ameren Missouri’s electric base rates would be frozen until April 1, 2020. Recoveries under the MEEIA, the FAC, and the RESRAM riders would not be frozen; however, except for costs recoverable under the MEEIA rider, Ameren Missouri would be unable to recover any amounts above the 2.85% cap from customers. If rate changes from the FAC or the RESRAM riders would cause rates to temporarily exceed the 2.85% cap, the overage would be deferred for future recovery in the next regulatory rate review; however, rates established in such regulatory rate review would be subject to the rate cap. Any deferred overages approved for recovery would be subject to deferral and recovered in a manner consistent with costs recovered under PISA. Both the rate cap and PISA election would be effective through December 2023, unless Ameren Missouri requests and receives MoPSC approval of an extension through December 2028. Ameren Missouri’s expected PISA election will support Ameren Missouri's ability to invest approximately $1 billion of incremental capital over the 2019 to 2023 period to strengthen and modernize Missouri's electric grid.

MoPSC Federal Income Tax Proceedings

In February 2018, the MoPSC initiated proceedings to investigate how the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA should be reflected in rates paid by customers of Missouri’s regulated utilities, including rates paid by electric and natural gas customers of Ameren Missouri. The proceeding for Ameren Missouri’s electric service business was dismissed after Missouri Senate Bill 564 was enacted on June 1, 2018, but the proceeding is still pending for Ameren Missouri’s natural gas distribution business. As of June 30, 2018, the potential reduction in natural gas customer rates is immaterial. The MoPSC is under no deadline to issue an order in the natural gas proceeding.

Wind Generation Facility and RESRAM

In the second quarter of 2018, Ameren Missouri entered into an agreement with a subsidiary of Terra-Gen, LLC to acquire a 400-megawatt wind generation facility after construction. The facility is expected to be located in northeastern Missouri and to be completed in 2020. The acquisition is subject to certain conditions, including the issuance of a certificate of convenience and necessity by the MoPSC, obtaining a MISO transmission interconnection agreement, and approval by the FERC. Ameren Missouri has filed for the certificate of convenience and necessity with the MoPSC. This facility would help Ameren Missouri to comply with the state renewable energy standard. In addition, Ameren Missouri requested the MoPSC to authorize a proposed RESRAM that would allow Ameren Missouri to adjust customer rates, including recovery of interest at a short-term borrowing rate, on an annual basis without a traditional regulatory rate review. The RESRAM is designed to mitigate the impacts of regulatory lag for investments in wind generation and other renewables by providing more timely recovery of costs and would provide Ameren Missouri a greater opportunity to earn its allowed return on investment. Ameren Missouri anticipates a decision by January 2019 related to the certificate of convenience and necessity and proposed RESRAM.

Renewable Choice Program

In June 2018, the MoPSC approved Ameren Missouri’s Renewable Choice Program, which allows large commercial and industrial customers and municipalities to receive up to 100 percent of their energy from renewable resources. The tariff-based program is designed to recover the costs of the election, net of changes in the market price of such energy. Based on customer contracts, the program enables Ameren Missouri to supply up to 400 megawatts of renewable wind energy generation, up to 200 megawatts of which it could own. As applicable, the addition of generation by Ameren Missouri would be subject to the issuance of a certificate of convenience and necessity by the MoPSC, obtaining transmission interconnection agreements with the MISO or other RTOs, and approval by the FERC. This generation would be incremental to the expected renewable generation included in the 2017 IRP. Without extension, the option to elect into the program will terminate in the third quarter of 2023.

MEEIA

In June 2018, Ameren Missouri filed a proposed customer energy-efficiency plan with the MoPSC under the MEEIA. This filing proposed a six-year plan, which includes a portfolio of customer energy-efficiency programs, along with a cost recovery mechanism. If the plan is approved, beginning in March 2019, Ameren Missouri intends to invest an average of $92 million per program year in the proposed customer energy-efficiency programs. Ameren Missouri requested continued use of a MEEIA rider, which allows Ameren Missouri to collect from or refund to customers any difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs and its lost revenues. In addition, Ameren Missouri requested incentives to earn additional revenues by achieving certain customer energy-efficiency goals, increasing from $10 million to $24 million annually, for a total of $115 million over the six-year period if 100% of its annual customer energy-efficiency goals are achieved. A decision by the MoPSC in this proceeding is anticipated by the first quarter of 2019.

The MEEIA 2016 program provided Ameren Missouri with a performance incentive to earn additional revenues by achieving certain customer energy-efficiency goals, including $27 million if 100% of the goals were achieved during the three-year period beginning March 2016, with the potential to earn more if Ameren Missouri’s energy savings exceeded those goals. In September 2017, Ameren Missouri received an order from the MoPSC approving Ameren Missouri’s energy savings results for the first year of the MEEIA 2016 programs. As a result of this order and in accordance with revenue recognition guidance, Ameren Missouri recognized $5 million of revenues in the first quarter of 2018 relating to the MEEIA 2016 performance incentive.

In July 2018, the Missouri Supreme Court overturned a 2016 decision by the Missouri Court of Appeals, Western District, which had upheld a 2015 MoPSC order regarding the determination of a certain input used to calculate the MEEIA 2013 performance incentive, and remanded the matter to the MoPSC. The MoPSC is required to issue a revised order consistent with the Missouri Supreme Court’s ruling; however, there is no deadline to issue such order. Upon issuance of the order, Ameren Missouri expects to recognize an additional $9 million MEEIA 2013 performance incentive.

Illinois

Electric Distribution Service Rates

In April 2018, Ameren Illinois filed its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2019 rates with the ICC. In July 2018, the ICC staff submitted its calculation of the revenue requirement included in Ameren Illinois’ update filing, recommending an amount comparable to Ameren Illinois’ filing. Pending ICC approval, this update filing will result in a $72 million increase in Ameren Illinois’ electric distribution service rates beginning in January 2019. This update reflects an increase to the annual formula rate based on 2017 actual costs and expected net plant additions for 2018 and an increase to include the 2017 revenue requirement reconciliation adjustment. It also includes a decrease for the conclusion of the 2016 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2018, consistent with the ICC’s December 2017 annual update filing order. An ICC decision in this proceeding is expected by December 2018. As of June 30, 2018, Ameren Illinois had recorded a regulatory asset of $62 million to reflect the difference between Ameren Illinois’ estimate of its 2018 revenue requirement and the revenue requirement reflected in customer rates, including interest.

Electric Customer Energy-Efficiency Investments

In June 2018, Ameren Illinois filed its annual electric customer energy-efficiency formula rate update to establish the revenue requirement to be used for 2019 rates with the ICC. Pending ICC approval, this update filing will result in 2019 rates for electric customer energy-efficiency investments of $34 million, which represents an increase of $20 million from the 2018 rates. An ICC decision regarding the revenue requirement to be used for customer rates in 2019 is expected by December 2018.

Income Tax Regulatory Mechanisms

In February 2018, the ICC granted Ameren Illinois’ request, filed in January 2018, to establish a rider to reduce Ameren Illinois’ electric distribution customer rates for the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA and the return of excess deferred taxes, net of the increase in state income taxes enacted in July 2017. Ameren Illinois' electric distribution customer rates were reduced as a result of the rider beginning in the first quarter of 2018. The estimated reduction of $50 million per year will continue through 2019, as base rates will reflect the current income tax rates starting in 2020.

In April 2018, the ICC approved a rider for the difference between revenues billed under natural gas rates established pursuant to Ameren Illinois’ most recent natural gas rate order, and the revenues that would have been billed had the state and federal tax rate changes discussed above been in effect. The rider required Ameren Illinois to record this regulatory liability beginning January 25, 2018. Ameren Illinois’ natural gas customer rates were reduced as a result of the rider beginning in May 2018, with an estimated reduction of up to $17 million, substantially over a one-year period.

2018 Natural Gas Delivery Service Regulatory Rate Review

In January 2018, Ameren Illinois filed a request with the ICC seeking approval to increase its annual rates for natural gas delivery service. In July 2018, Ameren Illinois and the ICC staff filed a stipulation and agreement with the ICC that, pending ICC approval, would result in an annual natural gas rate increase of $37 million, based on the terms of the agreement and subject to adjustments for updated rate case and other postretirement benefit expenses. This increase in annual rates includes a 9.87% return on common equity, a capital structure composed of 50% common equity, and a rate base of $1.6 billion. It also reflects the reduction in the federal corporate income tax rate as a result of the TCJA, as well as the increase in the Illinois corporate income tax rate that became effective in July 2017, which decreased the annual rates by approximately $17 million. In an attempt to reduce regulatory lag, Ameren Illinois used a 2019 future test year in this proceeding.

A decision by the ICC in this proceeding is required by December 2018, with new rates expected to be effective in January 2019. Ameren Illinois cannot predict the level of any delivery service rate changes the ICC may approve, nor whether any rate changes that may eventually be approved will be sufficient to enable Ameren Illinois to recover its costs and to earn a reasonable return on investments when the rate changes go into effect.

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 2016, the FERC issued a final order in the November 2013 complaint case, which lowered the allowed base return on common equity to 10.32%, or a 10.82% total allowed return on common equity with the inclusion of a 50 basis point incentive adder for participation in an RTO, effective since September 2016. The 10.82% allowed return on common equity may be replaced prospectively after the FERC issues a final order in the February 2015 complaint case, discussed below.

Since 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. MISO transmission owners subsequently filed a motion to dismiss the February 2015 complaint, as discussed below. The February 2015 complaint case seeks a further reduction in the allowed base return on common equity for 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. If approved by the FERC, it 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 a 50 basis point incentive adder for participation in an RTO. It would also require customer refunds, with interest, for that 15-month period. A final FERC order would also establish the allowed return on common equity that will apply prospectively from the effective date of such order, replacing the current 10.82% total return on common equity. In the second quarter of 2017, the United States Court of Appeals for the District of Columbia Circuit 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 the FERC is under no deadline to issue a final order, the timing of the issuance of the final order in the February 2015 complaint case is uncertain.

In September 2017, MISO transmission owners, including Ameren Missouri, Ameren Illinois, and ATXI, filed a motion to dismiss the February 2015 complaint case with the FERC. The MISO transmission owners maintain that the February 2015 complaint was predicated on the now superseded 12.38% allowed base return on common equity and is therefore inapplicable given the current 10.32% allowed base return on common equity. The MISO transmission owners further maintain that the current 10.32% allowed base return on common equity has not been proven to be unjust and unreasonable based on information provided, including the base return on common equity methodology ranges set forth in the February 2015 complaint case and in the initial decision issued by an administrative law judge in June 2016. Additionally, the MISO transmission owners maintain that the February 2015 complaint should be dismissed because the approach utilized in the case to assert that a return on common equity was unjust and unreasonable was insufficient. That same approach was rejected by the United States Court of Appeals for the District of Columbia Circuit, as discussed above. The FERC is under no deadline to issue an order on this motion.

As of June 30, 2018, Ameren and Ameren Illinois had recorded current regulatory liabilities of $43 million and $25 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.

FERC Federal Income Tax Proceeding and Formula Rate Change

In March 2018, the FERC granted a request filed in February 2018 by MISO transmission owners with forward-looking rate formulas, including Ameren Illinois and ATXI, to allow revisions to their 2018 electric transmission rates to reflect the effect of the reduction in federal income taxes enacted under the TCJA. Ameren Illinois and ATXI’s 2018 electric transmission rates have been reduced by $27 million and $23 million, respectively.

In May 2018, the FERC accepted Ameren Illinois and ATXI tariff filings to change the formula rate calculation. The change allows for the recovery or refund of both excess deferred income taxes resulting from tax law or rate changes and effect of permanent income tax differences and will be reflected in Ameren Illinois and ATXI’s electric transmission rates starting in January 2019.