Rate And Regulatory Matters	9 Months Ended
Sep. 30, 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 authorized 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 decreased 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 the third quarter of 2017, ATXI finalized an alternative project route and reached agreements with a cooperative electric company in northeast Missouri and Ameren Missouri to locate nearly all of the Mark Twain project on existing transmission line corridors. It also received assents for road crossings from the five affected counties in northeast Missouri. ATXI had previously filed suit in the circuit courts to obtain assents for the original project route. ATXI has since withdrawn one of the lawsuits. The other lawsuits remain pending but have been stayed until the first quarter of 2018. In September 2017, ATXI filed for a certificate of convenience and necessity with the MoPSC and anticipates a decision from the MoPSC in the first half of 2018. ATXI plans to complete the project in December 2019; however, delays in obtaining approval from the MoPSC could delay completion. Illinois IEIMA & FEJA Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity under a formula ratemaking process effective through 2022. This 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 based on that year's actual recoverable costs incurred and investment return. As of September 30, 2017, Ameren Illinois had recorded regulatory assets of $24 million to reflect its 2016 revenue requirement reconciliation adjustment, which was included in the April 2017 formula rate update discussed below, and $16 million for the approved 2015 revenue requirement reconciliation adjustment, each with interest. As of September 30, 2017, Ameren Illinois had recorded a regulatory liability of $1 million to reflect the difference between Ameren Illinois’ estimate of its 2017 revenue requirement and the revenue requirement reflected in customer rates, including interest. 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. In June 2017, the ICC staff submitted its calculation of the revenue requirement, which Ameren Illinois supported in its revised July 2017 filing, and recommended a decrease to the electric distribution service revenue requirement. Pending ICC approval, this update filing will result in a $17 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, as well as an increase to include the 2016 revenue requirement reconciliation adjustment. The increases in the update filing are more than offset by 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. In November 2017, an administrative law judge issued a proposed order that was consistent with Ameren Illinois’ revised July 2017 filing. 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 now recognizes revenue consistent with the timing of actual incurred electric distribution recoverable costs and recognizes revenue associated with the expected return on its rate base ratably over the year. Ameren Illinois recognized a reduction to electric revenue to reflect the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates of $76 million and $1 million for the three and nine months ended September 30, 2017, respectively. Comparative electric revenues at Ameren Illinois for the three and nine months ended September 30, 2016, were increased $11 million and $24 million, respectively, for the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates. In June 2017, pursuant to the FEJA, Ameren Illinois filed with the ICC an energy efficiency plan for 2018 through 2021. In September 2017, the ICC issued an order approving Ameren Illinois' implementation of FEJA electric energy efficiency savings targets and investments. Ameren Illinois plans to invest up to $99 million in electric energy efficiency programs per year from 2018 through 2021 that will earn a return. The electric energy efficiency program investments and the return on those investments will be collected from customers through a rider and will not be included in the IEIMA formula ratemaking process. 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 segment in the Illinois Rivers project, which has an estimated segment cost of approximately $85 million, of which $32 million was invested as of September 30, 2017. These cases had been filed in order to obtain necessary easements and rights of way to complete the segment. The court found that required notice was not given to the relevant landowners during the underlying ICC proceeding. ATXI intends to appeal this decision. ATXI plans to complete the project in 2019; however, delays associated with the condemnation proceedings or an appeal arising from the order dismissing the Edgar County cases could delay the completion date. The other eight segments of the Illinois Rivers project 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 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 a 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. In the first six months 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. 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 have since filed a motion to dismiss the February 2015 complaint. See below for additional information about the motion. 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, 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 a 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 is uncertain for two reasons. First, while the FERC reestablished a quorum of commissioners in August 2017 after six months without a quorum, the FERC is under no deadline to issue a final order. Second, 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. 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 being an unjust and unreasonable return and is not applicable given the currently effective 10.32% allowed base return on common equity. The MISO transmission owners further maintain that the currently effective 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 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 is insufficient. That same approach was rejected by the United States Court of Appeals for the District of Columbia Circuit, as discussed above. FERC is under no deadline to issue an order on this motion. As of September 30, 2017, Ameren and Ameren Illinois had 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 authorized 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 decreased 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 the third quarter of 2017, ATXI finalized an alternative project route and reached agreements with a cooperative electric company in northeast Missouri and Ameren Missouri to locate nearly all of the Mark Twain project on existing transmission line corridors. It also received assents for road crossings from the five affected counties in northeast Missouri. ATXI had previously filed suit in the circuit courts to obtain assents for the original project route. ATXI has since withdrawn one of the lawsuits. The other lawsuits remain pending but have been stayed until the first quarter of 2018. In September 2017, ATXI filed for a certificate of convenience and necessity with the MoPSC and anticipates a decision from the MoPSC in the first half of 2018. ATXI plans to complete the project in December 2019; however, delays in obtaining approval from the MoPSC could delay completion.

Illinois

IEIMA & FEJA

Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity under a formula ratemaking process effective through 2022. This 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 based on that year's actual recoverable costs incurred and investment return. As of September 30, 2017, Ameren Illinois had recorded regulatory assets of $24 million to reflect its 2016 revenue requirement reconciliation adjustment, which was included in the April 2017 formula rate update discussed below, and $16 million for the approved 2015 revenue requirement reconciliation adjustment, each with interest. As of September 30, 2017, Ameren Illinois had recorded a regulatory liability of $1 million to reflect the difference between Ameren Illinois’ estimate of its 2017 revenue requirement and the revenue requirement reflected in customer rates, including interest.

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. In June 2017, the ICC staff submitted its calculation of the revenue requirement, which Ameren Illinois supported in its revised July 2017 filing, and recommended a decrease to the electric distribution service revenue requirement. Pending ICC approval, this update filing will result in a $17 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, as well as an increase to include the 2016 revenue requirement reconciliation adjustment. The increases in the update filing are more than offset by 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. In November 2017, an administrative law judge issued a proposed order that was consistent with Ameren Illinois’ revised July 2017 filing. 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 now recognizes revenue consistent with the timing of actual incurred electric distribution recoverable costs and recognizes revenue associated with the expected return on its rate base ratably over the year. Ameren Illinois recognized a reduction to electric revenue to reflect the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates of $76 million and $1 million for the three and nine months ended September 30, 2017, respectively. Comparative electric revenues at Ameren Illinois for the three and nine months ended September 30, 2016, were increased $11 million and $24 million, respectively, for the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates.

In June 2017, pursuant to the FEJA, Ameren Illinois filed with the ICC an energy efficiency plan for 2018 through 2021. In September 2017, the ICC issued an order approving Ameren Illinois' implementation of FEJA electric energy efficiency savings targets and investments. Ameren Illinois plans to invest up to $99 million in electric energy efficiency programs per year from 2018 through 2021 that will earn a return. The electric energy efficiency program investments and the return on those investments will be collected from customers through a rider and will not be included in the IEIMA formula ratemaking process.

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 segment in the Illinois Rivers project, which has an estimated segment cost of approximately $85 million, of which $32 million was invested as of September 30, 2017. These cases had been filed in order to obtain necessary easements and rights of way to complete the segment. The court found that required notice was not given to the relevant landowners during the underlying ICC proceeding. ATXI intends to appeal this decision. ATXI plans to complete the project in 2019; however, delays associated with the condemnation proceedings or an appeal arising from the order dismissing the Edgar County cases could delay the completion date. The other eight segments of the Illinois Rivers project 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 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 a 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. In the first six months 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. 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 have since filed a motion to dismiss the February 2015 complaint. See below for additional information about the motion. 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, 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 a 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 is uncertain for two reasons. First, while the FERC reestablished a quorum of commissioners in August 2017 after six months without a quorum, the FERC is under no deadline to issue a final order. Second, 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.

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 being an unjust and unreasonable return and is not applicable given the currently effective 10.32% allowed base return on common equity. The MISO transmission owners further maintain that the currently effective 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 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 is insufficient. That same approach was rejected by the United States Court of Appeals for the District of Columbia Circuit, as discussed above. FERC is under no deadline to issue an order on this motion.

As of September 30, 2017, Ameren and Ameren Illinois had 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.