Rate And Regulatory Matters	3 Months Ended
Mar. 31, 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 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. As of March 31, 2018, Ameren Missouri recorded an immaterial decrease in revenues as a regulatory liability for a potential reduction in customer rates. The MoPSC is under no deadline to issue an order in these proceedings. MEEIA 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, 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. Illinois Electric Distribution Service Rates Under a formula ratemaking framework effective through 2022, Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity. The formula ratemaking framework 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. As of March 31, 2018, Ameren Illinois had recorded regulatory assets of $52 million to reflect its 2017 revenue requirement reconciliation adjustment, which was included in the April 2018 formula rate update discussed below, and $19 million for the approved 2016 revenue requirement reconciliation adjustment, each with interest. As of March 31, 2018, Ameren Illinois had recorded a regulatory asset of $42 million to reflect the difference between Ameren Illinois’ estimate of its 2018 revenue requirement and the revenue requirement reflected in customer rates, including interest. In April 2018, Ameren Illinois filed with the ICC its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2019 rates. 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, an increase to include the 2017 revenue requirement reconciliation adjustment, and 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 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 through 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 net amount of the difference between revenues billed under natural gas rates in effect, 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. The rider calculates such differences by evaluating the return of excess deferred taxes and income taxes included in the revenue requirement prior to the reduction in the federal statutory corporate income tax rate enacted under the TCJA and the increase in state income taxes enacted in July 2017. Ameren Illinois’ natural gas customer rates were reduced through the rider beginning in May 2018, with an estimated reduction of up to $16 million reflected over a one-year period. As of March 31, 2018, Ameren Illinois recorded an immaterial reduction in revenues as a regulatory liability for an anticipated reduction in customer rates. 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 by $49 million. In the second quarter of 2018, Ameren Illinois and the ICC staff entered into agreements to use a 9.87% return on common equity and a capital structure composed of up to and including 50% common equity in this regulatory rate review. The return on common equity and the common equity ratio are subject to ICC approval. The impact of a 9.87% return on common equity would lower the requested annual natural gas rate increase to an estimated $44 million, which includes an estimated $42 million of annual rates that would otherwise be recovered under the QIP rider. This estimated increase in annual rates includes 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. 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 September 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 March 31, 2018, Ameren and Ameren Illinois had recorded current regulatory liabilities of $42 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 In March 2018, the FERC granted a request filed in February 2018 by MISO transmission owners with forward-looking rate formulas, which included 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.

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

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. As of March 31, 2018, Ameren Missouri recorded an immaterial decrease in revenues as a regulatory liability for a potential reduction in customer rates. The MoPSC is under no deadline to issue an order in these proceedings.

MEEIA

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, 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.

Illinois

Electric Distribution Service Rates

Under a formula ratemaking framework effective through 2022, Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity. The formula ratemaking framework 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. As of March 31, 2018, Ameren Illinois had recorded regulatory assets of $52 million to reflect its 2017 revenue requirement reconciliation adjustment, which was included in the April 2018 formula rate update discussed below, and $19 million for the approved 2016 revenue requirement reconciliation adjustment, each with interest. As of March 31, 2018, Ameren Illinois had recorded a regulatory asset of $42 million to reflect the difference between Ameren Illinois’ estimate of its 2018 revenue requirement and the revenue requirement reflected in customer rates, including interest.

In April 2018, Ameren Illinois filed with the ICC its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2019 rates. 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, an increase to include the 2017 revenue requirement reconciliation adjustment, and 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 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 through 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 net amount of the difference between revenues billed under natural gas rates in effect, 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. The rider calculates such differences by evaluating the return of excess deferred taxes and income taxes included in the revenue requirement prior to the reduction in the federal statutory corporate income tax rate enacted under the TCJA and the increase in state income taxes enacted in July 2017. Ameren Illinois’ natural gas customer rates were reduced through the rider beginning in May 2018, with an estimated reduction of up to $16 million reflected over a one-year period. As of March 31, 2018, Ameren Illinois recorded an immaterial reduction in revenues as a regulatory liability for an anticipated reduction in customer rates.

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 by $49 million. In the second quarter of 2018, Ameren Illinois and the ICC staff entered into agreements to use a 9.87% return on common equity and a capital structure composed of up to and including 50% common equity in this regulatory rate review. The return on common equity and the common equity ratio are subject to ICC approval. The impact of a 9.87% return on common equity would lower the requested annual natural gas rate increase to an estimated $44 million, which includes an estimated $42 million of annual rates that would otherwise be recovered under the QIP rider. This estimated increase in annual rates includes 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. 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 September 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 March 31, 2018, Ameren and Ameren Illinois had recorded current regulatory liabilities of $42 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

In March 2018, the FERC granted a request filed in February 2018 by MISO transmission owners with forward-looking rate formulas, which included 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.