Rate And Regulatory Matters	3 Months Ended
Mar. 31, 2019
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 Wind Generation Facilities and RESRAM In May 2019, Ameren Missouri entered into a build-transfer agreement with a subsidiary of Enel Green Power North America, Inc. to acquire, after construction, an up-to 300-megawatt wind generation facility to be located in northwestern Missouri. Ameren Missouri expects to file for a certificate of convenience and necessity with the MoPSC in May 2019. Final RTO interconnection costs are expected to be determined in June 2019, and a related RTO transmission interconnection agreement is expected in the fall of 2019. In 2018, Ameren Missouri entered into two build-transfer agreements to acquire, after construction, an up-to 400-megawatt wind generation facility and an up-to 157-megawatt wind generation facility. In October 2018, the MoPSC issued an order approving a unanimous stipulation and agreement regarding a requested certificate of convenience and necessity for the up-to 400-megawatt facility. In March 2019, the MoPSC issued an order approving a nonunanimous stipulation and agreement regarding a requested certificate of convenience and necessity for the up-to 157-megawatt facility. Final MISO interconnection costs for the up-to 400-megawatt facility are expected to be determined in June 2019, and a related transmission interconnection agreement with the MISO is expected in the fall of 2019. Final MISO interconnection costs for the up-to 157-megawatt facility are expected to be determined in the fall of 2019, and a related transmission interconnection agreement with the MISO is expected in early 2020. All three facilities are expected to be completed by the end of 2020, which would support Ameren Missouri’s compliance with the Missouri renewable energy standard. Each acquisition is subject to certain conditions, including the issuance of a certificate of convenience and necessity by the MoPSC for the up-to 300-megawatt facility, obtaining FERC approval for the up-to 157-megawatt facility and the up-to 300-megawatt facility, entering into an RTO transmission interconnection agreement at an acceptable cost for each facility, and other customary contract terms and conditions. The three build-transfer agreements collectively represent approximately $1.4 billion of capital expenditures expected in 2020. In January 2019, the MoOPC filed an appeal with the Missouri Court of Appeals, Western District, challenging the MoPSC’s December 2018 order allowing Ameren Missouri to recover, through the RESRAM, the 15% of depreciation expense and weighted average cost of capital return not recovered under PISA. Ameren Missouri expects a decision by the end of 2019. The RESRAM is designed to mitigate the impacts of regulatory lag for the cost of compliance with renewable energy standards, including recovery of investments in wind and other renewable generation, by providing more timely recovery of costs and a return on investments not already provided for in customer rates or recovered under PISA. RESRAM regulatory assets earn carrying costs at short-term interest rates. MEEIA As a result of MoPSC orders issued in September 2017, October 2018, and January 2019 related to performance incentives for the MEEIA 2013 and MEEIA 2016 programs, Ameren Missouri recognized gains of $20 million and $5 million during the three months ended March 31, 2019 and 2018, respectively. 2018 Natural Gas Delivery Service Regulatory Rate Review In December 2018, Ameren Missouri filed a request with the MoPSC to increase its annual revenues for natural gas delivery service by approximately $4 million, prior to the interim rate reduction discussed below. The natural gas delivery service rate increase request is based on a 10.30% return on equity, a capital structure composed of 51.84% common equity, a rate base of $259 million, and a test year ended June 30, 2018, with certain pro-forma adjustments through the anticipated true-up date of May 31, 2019. In December 2018, the MoPSC issued an order approving a stipulation and agreement for an interim rate reduction of $2 million to reflect cost of service updates including the reduction in the federal corporate income tax rate and the amortization of excess deferred taxes as a result of the TCJA. As a result of the interim rate reduction, Ameren Missouri's requested annual revenues reflect a $6 million increase from interim rates. The interim rate reduction became effective January 2, 2019, and will continue until new rates are approved by the MoPSC in this regulatory rate review. In April 2019, the MoPSC staff recommended a $1 million increase to interim rates, based on a 9.5% return on equity and a 50% ceiling on the common equity ratio. Illinois Electric Distribution Service Rates In April 2019, Ameren Illinois filed its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2020 rates with the ICC. Pending ICC approval, this update filing will result in a $7 million decrease in Ameren Illinois’ electric distribution service rates, beginning in January 2020. This update reflects an increase to the annual formula rate based on 2018 actual costs and expected net plant additions for 2019, and an increase to include the 2018 revenue requirement reconciliation adjustment. It also reflects a decrease for the conclusion of the 2017 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2019, consistent with the ICC’s November 2018 annual update filing order. An ICC decision in this proceeding is expected by December 2019. As of March 31, 2019, Ameren Illinois had recorded a regulatory asset of $41 million to reflect the difference between Ameren Illinois’ estimate of its 2019 revenue requirement and the revenue requirement reflected in customer rates, including interest. ATXI’s Illinois Rivers Project In August 2017, the Illinois Circuit Court for Edgar County dismissed several of ATXI’s condemnation cases related to one line segment in the Illinois Rivers project. These cases had been filed to obtain easements and rights of way necessary to complete the line segment. The court found that required notice was not given to the relevant landowners during the underlying ICC proceeding. Upon appeal, in October 2018, the Illinois Supreme Court reversed the Illinois Circuit Court for Edgar County’s decision and remanded the case for further proceedings. In December 2018, the Illinois Supreme Court issued an order to stay its October 2018 ruling. In February 2019, the landowners filed an appeal with the United States Supreme Court, which was denied in April 2019. In the second quarter of 2019, ATXI intends to seek to reinstate the condemnation cases that were previously dismissed. ATXI expects to complete the line segment in 2020. The estimated line segment capital expenditure investment is approximately $81 million, of which $38 million was invested as of March 31, 2019. The other eight line segments of the Illinois Rivers project have already been completed and placed in-service and are not affected by these proceedings. 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 an 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 April 2017, the United States Court of Appeals for the District of Columbia Circuit vacated and remanded to the FERC an order in an unrelated case in which the FERC established the allowed base return on common equity methodology subsequently used in the two MISO complaint cases described above. In October 2018, the FERC issued an order in an unrelated case that proposed a new methodology for determining the base return on equity and required further briefs from the participants. In November 2018, the FERC issued an order related to the February 2015 complaint case and the September 2016 order, which required participants to file briefs in February 2019 regarding the FERC’s proposed methodology for determining the base return on common equity, including whether and how to apply the proposed methodology to the two MISO complaint cases. In March 2019, the FERC issued separate Notices of Inquiry regarding its allowed base return on common equity policy and its transmission incentives policy, with comments due in June 2019. The Notice of Inquiry addressing the FERC’s return on common equity policy, among other things, broadens the ability to comment on the new methodology beyond electric utilities that are participants in the complaint cases, and the transmission incentives Notice of Inquiry is open for industry comment on the FERC’s transmission incentive policy, including incentive adders to the return on common equity. Ameren is unable to predict the ultimate impact of the proposed methodology on these complaint cases or the Notices of Inquiry at this time. As the FERC is under no deadline to issue a final order, the timing of the final order in the February 2015 complaint case, and any potential impact to the amounts refunded as a result of the September 2016 order, 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 in an unrelated case, as discussed above. The FERC is under no deadline to issue an order on this motion. As of March 31, 2019, Ameren and Ameren Illinois had recorded current regulatory liabilities of $44 million and $26 million, respectively, to reflect the expected refunds, including interest, associated with the reduced allowed return 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

Wind Generation Facilities and RESRAM

In May 2019, Ameren Missouri entered into a build-transfer agreement with a subsidiary of Enel Green Power North America, Inc. to acquire, after construction, an up-to 300-megawatt wind generation facility to be located in northwestern Missouri. Ameren Missouri expects to file for a certificate of convenience and necessity with the MoPSC in May 2019. Final RTO interconnection costs are expected to be determined in June 2019, and a related RTO transmission interconnection agreement is expected in the fall of 2019.

In 2018, Ameren Missouri entered into two build-transfer agreements to acquire, after construction, an up-to 400-megawatt wind generation facility and an up-to 157-megawatt wind generation facility. In October 2018, the MoPSC issued an order approving a unanimous stipulation and agreement regarding a requested certificate of convenience and necessity for the up-to 400-megawatt facility. In March 2019, the MoPSC issued an order approving a nonunanimous stipulation and agreement regarding a requested certificate of convenience and necessity for the up-to 157-megawatt facility. Final MISO interconnection costs for the up-to 400-megawatt facility are expected to be determined in June 2019, and a related transmission interconnection agreement with the MISO is expected in the fall of 2019. Final MISO interconnection costs for the up-to 157-megawatt facility are expected to be determined in the fall of 2019, and a related transmission interconnection agreement with the MISO is expected in early 2020.

All three facilities are expected to be completed by the end of 2020, which would support Ameren Missouri’s compliance with the Missouri renewable energy standard. Each acquisition is subject to certain conditions, including the issuance of a certificate of convenience and necessity by the MoPSC for the up-to 300-megawatt facility, obtaining FERC approval for the up-to 157-megawatt facility and the up-to 300-megawatt facility, entering into an RTO transmission interconnection agreement at an acceptable cost for each facility, and other customary contract terms and conditions.

The three build-transfer agreements collectively represent approximately $1.4 billion of capital expenditures expected in 2020.

In January 2019, the MoOPC filed an appeal with the Missouri Court of Appeals, Western District, challenging the MoPSC’s December 2018 order allowing Ameren Missouri to recover, through the RESRAM, the 15% of depreciation expense and weighted average cost of capital return not recovered under PISA. Ameren Missouri expects a decision by the end of 2019. The RESRAM is designed to mitigate the impacts of regulatory lag for the cost of compliance with renewable energy standards, including recovery of investments in wind and other renewable generation, by providing more timely recovery of costs and a return on investments not already provided for in customer rates or recovered under PISA. RESRAM regulatory assets earn carrying costs at short-term interest rates.

MEEIA

As a result of MoPSC orders issued in September 2017, October 2018, and January 2019 related to performance incentives for the MEEIA 2013 and MEEIA 2016 programs, Ameren Missouri recognized gains of $20 million and $5 million during the three months ended March 31, 2019 and 2018, respectively.

2018 Natural Gas Delivery Service Regulatory Rate Review

In December 2018, Ameren Missouri filed a request with the MoPSC to increase its annual revenues for natural gas delivery service by approximately $4 million, prior to the interim rate reduction discussed below. The natural gas delivery service rate increase request is based on a 10.30% return on equity, a capital structure composed of 51.84% common equity, a rate base of $259 million, and a test year ended June 30, 2018, with certain pro-forma adjustments through the anticipated true-up date of May 31, 2019. In December 2018, the MoPSC issued an order approving a stipulation and agreement for an interim rate reduction of $2 million to reflect cost of service updates including the reduction in the federal corporate income tax rate and the amortization of excess deferred taxes as a result of the TCJA. As a result of the interim rate reduction, Ameren Missouri's requested annual revenues reflect a $6 million increase from interim rates. The interim rate reduction became effective January 2, 2019, and will continue until new rates are approved by the MoPSC in this regulatory rate review. In April 2019, the MoPSC staff recommended a $1 million increase to interim rates, based on a 9.5% return on equity and a 50% ceiling on the common equity ratio.

Illinois

Electric Distribution Service Rates

In April 2019, Ameren Illinois filed its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2020 rates with the ICC. Pending ICC approval, this update filing will result in a $7 million decrease in Ameren Illinois’ electric distribution service rates, beginning in January 2020. This update reflects an increase to the annual formula rate based on 2018 actual costs and expected net plant additions for 2019, and an increase to include the 2018 revenue requirement reconciliation adjustment. It also reflects a decrease for the conclusion of the 2017 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2019, consistent with the ICC’s November 2018 annual update filing order. An ICC decision in this proceeding is expected by December 2019. As of March 31, 2019, Ameren Illinois had recorded a regulatory asset of $41 million to reflect the difference between Ameren Illinois’ estimate of its 2019 revenue requirement and the revenue requirement reflected in customer rates, including interest.

ATXI’s Illinois Rivers Project

In August 2017, the Illinois Circuit Court for Edgar County dismissed several of ATXI’s condemnation cases related to one line segment in the Illinois Rivers project. These cases had been filed to obtain easements and rights of way necessary to complete the line segment. The court found that required notice was not given to the relevant landowners during the underlying ICC proceeding. Upon appeal, in October 2018, the Illinois Supreme Court reversed the Illinois Circuit Court for Edgar County’s decision and remanded the case for further proceedings. In December 2018, the Illinois Supreme Court issued an order to stay its October 2018 ruling. In February 2019, the landowners filed an appeal with the United States Supreme Court, which was denied in April 2019. In the second quarter of 2019, ATXI intends to seek to reinstate the condemnation cases that were previously dismissed. ATXI expects to complete the line segment in 2020. The estimated line segment capital expenditure investment is approximately $81 million, of which $38 million was invested as of March 31, 2019. The other eight line segments of the Illinois Rivers project have already been completed and placed in-service and are not affected by these proceedings.

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 an 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 April 2017, the United States Court of Appeals for the District of Columbia Circuit vacated and remanded to the FERC an order in an unrelated case in which the FERC established the allowed base return on common equity methodology subsequently used in the two MISO complaint cases described above. In October 2018, the FERC issued an order in an unrelated case that proposed a new methodology for determining the base return on equity and required further briefs from the participants. In November 2018, the FERC issued an order related to the February 2015 complaint case and the September 2016 order, which required participants to file briefs in February 2019 regarding the FERC’s proposed methodology for determining the base return on common equity, including whether and how to apply the proposed methodology to the two MISO complaint cases. In March 2019, the FERC issued separate Notices of Inquiry regarding its allowed base return on common equity policy and its transmission incentives policy, with comments due in June 2019. The Notice of Inquiry addressing the FERC’s return on common equity policy, among other things, broadens the ability to comment on the new methodology beyond electric utilities that are participants in the complaint cases, and the transmission incentives Notice of Inquiry is open for industry comment on the FERC’s transmission incentive policy, including incentive adders to the return on common equity. Ameren is unable to predict the ultimate impact of the proposed methodology on these complaint cases or the Notices of Inquiry at this time. As the FERC is under no deadline to issue a final order, the timing of the final order in the February 2015 complaint case, and any potential impact to the amounts refunded as a result of the September 2016 order, 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 in an unrelated case, as discussed above. The FERC is under no deadline to issue an order on this motion.

As of March 31, 2019, Ameren and Ameren Illinois had recorded current regulatory liabilities of $44 million and $26 million, respectively, to reflect the expected refunds, including interest, associated with the reduced allowed return 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.