Regulation	12 Months Ended
Dec. 31, 2017
Regulated Operations [Abstract]
Regulation	Regulation General The rates and services of the Company are regulated by incorporated municipalities in Texas, the PUCT, the NMPRC and the FERC. Municipal orders, ordinances and other agreements regarding rates and services adopted by Texas municipalities are subject to review and approval by the PUCT. The FERC has jurisdiction over the Company's wholesale (sales for resale) transactions, transmission service and compliance with federally-mandated reliability standards. The decisions of the PUCT, the NMPRC and the FERC are subject to judicial review. Texas Regulatory Matters 2015 Texas Retail Rate Case Filing. On August 10, 2015, the Company filed with the City of El Paso, other municipalities incorporated in its Texas service territory, and the PUCT in Docket No. 44941, a request for an annual increase in non-fuel base revenues ("2015 Texas Retail Rate Case"). On July 21, 2016, the parties to PUCT Docket No. 44941 filed the Joint Motion to Implement Uncontested Amended and Restated Stipulation and Agreement which was unopposed by the parties (the "2016 Unopposed Settlement"). On August 25, 2016, the PUCT approved the 2016 Unopposed Settlement and issued the 2016 PUCT Final Order, as proposed. The 2016 PUCT Final Order provided for: (i) an annual non-fuel base rate increase, lower annual depreciation expense, a revised return on equity for AFUDC purposes, and the inclusion of substantially all new plant in service in rate base; (ii) an additional annual non-fuel base rate increase of $3.7 million related to Four Corners Generating Station ("Four Corners") costs, which was collected through a surcharge that terminated on July 11, 2017; (iii) removing the separate rate treatment for residential customers with solar systems that the Company had proposed in its August 10, 2015 filing; (iv) allowing the Company to recover $3.1 million in rate case expenses through a separate surcharge; and (v) allowing the Company to recover revenues associated with the relate back of rates to consumption on and after January 12, 2016 through March 31, 2016 through a separate surcharge. Interim rates associated with the annual non-fuel base rate increase became effective on April 1, 2016. The additional surcharges associated with the incremental Four Corners costs, rate case expenses and the relate back of rates to consumption on and after January 12, 2016 through March 31, 2016 were implemented on October 1, 2016. For financial reporting purposes, the Company deferred any recognition of the Company's request in its 2015 Texas Retail Rate Case until it received the 2016 PUCT Final Order on August 25, 2016. Accordingly, it reported in the third quarter of 2016 the cumulative effect of the 2016 PUCT Final Order, which related back to January 12, 2016. 2017 Texas Retail Rate Case Filing. On February 13, 2017, the Company filed with the City of El Paso, other municipalities incorporated in the Company's Texas service territory and the PUCT in Docket No. 46831, a request for an increase in non-fuel base revenues ("2017 Texas Retail Rate Case"). On November 2, 2017, the Company filed the Joint Motion to Implement Uncontested Stipulation and Agreement with the Administrative Law Judges for the 2017 Texas Retail Rate Case. On December 18, 2017, the PUCT issued its final order in the Company's rate case pending in Docket No. 46831 ("2017 PUCT Final Order"), which provides, among other things, for the following: (i) an annual non-fuel base rate increase of $14.5 million; (ii) a return on equity of 9.65%; (iii) all new plant in service as filed in the Company's rate filing package was prudent and used and useful and therefore is included in rate base; (iv) recovery of the costs of decommissioning Four Corners in the amount of $5.5 million over a seven year period beginning August 1, 2017; (v) the Company to recover reasonable rate case expenses of approximately $3.4 million through a separate surcharge over a three year period; and (vi) a requirement that the Company file a refund tariff if the federal statutory income tax rate, as it relates to the Company, is decreased before the Company files its next rate case. The 2017 PUCT Final Order also establishes baseline revenue requirements for recovery of future transmission and distribution investment costs, and includes a minimum monthly bill of $30.00 for new residential customers with distributed generation, such as private rooftop solar. Additionally, the 2017 PUCT Final Order allows for the annual recovery of $2.1 million of nuclear decommissioning funding and establishes annual depreciation expense that is approximately $1.9 million lower than the annual amount requested by the Company in its initial filing. Finally, the 2017 PUCT Final Order allows for the Company to recover revenues associated with the relate back of rates to consumption on and after July 18, 2017 through a separate surcharge. New base rates, including additional surcharges associated with rate case expenses and the relate back of rates to consumption on and after July 18, 2017 through December 31, 2017 were implemented in January 2018. For financial reporting purposes, the Company deferred any recognition of the Company's request in its 2017 Texas Retail Rate Case until it received the 2017 PUCT Final Order on December 18, 2017. Accordingly, it reported in the fourth quarter of 2017 the cumulative effect of the 2017 PUCT Final Order, which related back to July 18, 2017. The 2017 PUCT Final Order requires the Company to file a refund tariff if the federal statutory income tax rate, as it relates to the Company, is decreased before the Company files its next rate case. Following the enactment of the TCJA on December 22, 2017, and in compliance with the 2017 PUCT Final Order, the Company will reduce the recognition of Texas jurisdictional revenues beginning January 1, 2018, to approximate the tax savings resulting from the TCJA and will file a refund tariff which the Company will ask to be implemented in the first half of 2018. The refund tariff is expected to be reflected in rates over a period of a year and will be updated annually until new base rates are implemented pursuant to the Company's next rate case filing. See Part II, Item 8, Financial Statements and Supplementary Data, Note J for further details. Energy Efficiency Cost Recovery Factor. On May 1, 2017, the Company filed its annual application, which was assigned PUCT Docket No. 47125, to establish its energy efficiency cost recovery factor ("EECRF") for 2018. In addition to projected energy efficiency costs for 2018 and a true-up to prior year actual costs, the Company requested approval of an incentive bonus for the 2016 energy efficiency program results in accordance with PUCT rules. Interim rates were approved effective January 1, 2018. The Company, the staff of the PUCT, and the City of El Paso reached an agreement that includes an incentive bonus of $0.8 million. The agreement was filed on January 25, 2018, and was approved by the PUCT on February 15, 2018. Fuel and Purchased Power Costs. The Company's actual fuel costs, including purchased power energy costs, are recovered from customers through a fixed fuel factor. The PUCT has adopted a fuel cost recovery rule (the "Texas Fuel Rule") that allows the Company to seek periodic adjustments to its fixed fuel factor. The Company can seek to revise its fixed fuel factor based upon the approved formula at least four months after its last revision except in the month of December. The Texas Fuel Rule requires the Company to request to refund fuel costs in any month when the over-recovery balance exceeds a threshold material amount and it expects fuel costs to continue to be materially over-recovered. The Texas Fuel Rule also permits the Company to seek to surcharge fuel under-recoveries in any month the balance exceeds a threshold material amount and it expects fuel cost recovery to continue to be materially under-recovered. Fuel over and under-recoveries are considered material when they exceed 4% of the previous twelve months' fuel costs. All such fuel revenue and expense activities are subject to periodic final review by the PUCT in fuel reconciliation proceedings. On November 30, 2016, the Company filed a request, which was assigned PUCT Docket No. 46610, to increase its fixed fuel factor by approximately 28.8% to reflect increased fuel expenses primarily related to an increase in the price of natural gas used to generate power. The increase in the fixed fuel factor was effective on an interim basis January 1, 2017 and approved by the PUCT on January 10, 2017. As of September 30, 2017, the Company had over-recovered fuel costs in the amount of $1.1 million for the Texas jurisdiction. On October 13, 2017, the Company filed a request, which was assigned PUCT Docket No. 47692, to decrease the Texas fixed fuel factor by approximately 19% to reflect decreased fuel expenses primarily related to a decrease in the price of natural gas used to generate power. The decrease in the Texas fixed fuel factor became effective beginning with the November 2017 billing month and will continue thereafter until changed by the PUCT. At December 31, 2017, the Company had a net fuel over-recovery balance of approximately $5.8 million in Texas. Fuel Reconciliation Proceeding. On September 27, 2016, the Company filed an application with the PUCT, designated as PUCT Docket No. 46308, to reconcile $436.6 million of Texas fuel and purchased power expenses incurred during the period of April 1, 2013 through March 31, 2016. On June 29, 2017, the PUCT approved a settlement in this proceeding. The settlement provides for the reconciliation of fuel and purchased power costs incurred from April 1, 2013 through March 31, 2016. Additionally, the settlement modifies and tightens the Palo Verde performance rewards measurement bands beginning with the 2018 performance period. The financial results for the twelve months ended December 31, 2017 include a $5.0 million, pre-tax increase to income reflecting the settlement of the Texas fuel reconciliation proceeding. This amount represents Palo Verde performance rewards associated with the 2013 to 2015 performance periods net of disallowed fuel and purchased power costs as approved in the settlement. Texas jurisdictional fuel and purchased power costs subject to prudence review are costs from April 1, 2016 through December 31, 2017 that total approximately $250.9 million. Community Solar. On June 8, 2015, the Company filed a petition with the PUCT to initiate a community solar program that includes the construction and ownership of a 3 MW solar photovoltaic system located at the Company's Montana Power Station ("MPS"). Participation is on a voluntary basis, and customers contract for a set capacity (kW) amount and receive all energy produced. This case was assigned PUCT Docket No. 44800. The Company filed a settlement agreement among all parties on July 1, 2016 approving the program, and the PUCT approved the settlement agreement and program on September 1, 2016. On April 19, 2017, the Company announced that the entire 3 MW program was fully subscribed by approximately 1,500 Texas customers. The Community Solar facility began commercial operation on May 31, 2017. Four Corners Generating Station. On February 17, 2015, the Company and Arizona Public Service Company ("APS") entered into an asset purchase agreement (the "Purchase and Sale Agreement") providing for the sale of the Company's interest in Four Corners to APS. The sale of the Company's interest in Four Corners closed on July 6, 2016. See Part II, Item 8, Financial Statements and Supplementary Data, Note E for further details on the sale of Four Corners. On June 10, 2015, the Company filed an application in Texas requesting reasonableness and public interest findings and certain rate and accounting findings related to the Purchase and Sale Agreement. This case was assigned PUCT Docket No. 44805. Subsequent to the filing of the application, the case was subject to numerous procedural matters, including a March 23, 2016 order in which the PUCT determined not to dismiss the reasonableness and public interest issues in this docket but to consider the requested rate and accounting findings, including coal mine reclamation costs, in a rate case proceeding. On September 1, 2016, a motion by parties in the proceeding to suspend the procedural schedule in order to pursue settlement was approved. On March 3, 2017, the Company filed a Joint Motion to Implement Stipulation and Agreement (the "Stipulation and Agreement"), and PUCT Staff filed its recommendation that the Company’s disposition of its interest in Four Corners was reasonable and consistent with the public interest. Additionally, the signatories of the Stipulation and Agreement agreed to support the recovery of the Company's Four Corners decommissioning costs in the 2017 Texas Retail Rate Case. A final order approving the Stipulation and Agreement was adopted by the PUCT on March 30, 2017. The approval to recover Four Corners decommissioning costs was included in the 2017 PUCT Final Order. Other Required Approvals. The Company has obtained other required approvals for tariffs and other approvals required by the Texas Public Utility Regulatory Act and the PUCT. New Mexico Regulatory Matters 2015 New Mexico Rate Case Filing. On May 11, 2015, the Company filed a request with the NMPRC, in Case No. 15-00127-UT, for an annual increase in non-fuel base rates. On June 8, 2016, the NMPRC issued the NMPRC Final Order which approved an annual increase in non-fuel base rates of approximately $0.6 million, an increase of approximately $0.5 million in other service fees and a decrease in the Company's allowed return on equity to 9.48%. The NMPRC Final Order concluded that all of the Company's new plant in service was reasonable and necessary and therefore would be recoverable in rates. The Company's rates were approved by the NMPRC effective July 1, 2016 and implemented at such time. Future New Mexico Rate Case Filing. NMPRC Case No. 15-00109-UT required the Company to make a rate filing in New Mexico in the second quarter of 2017 using a historical test year ended December 31, 2016. On March 24, 2017, the Company, NMPRC Utility Division Staff and the New Mexico Attorney General filed a Joint Motion to Modify Filing Date Stated in Final Order requesting that the rate filing date be changed to no later than July 31, 2019, using the appropriate historical test year period. The joint request was approved by the NMPRC on April 12, 2017. The NMPRC has initiated an investigation into the impact of the TCJA on utility customers that may require earlier action by the Company. The Company is evaluating possible approaches to begin providing a refund credit for the TCJA income tax rate decrease to New Mexico customers. Fuel and Purchased Power Costs.Historically, fuel and purchased power costs were recovered through base rates and a Fuel and Purchased Power Cost Adjustment Clause (the "FPPCAC") that accounts for changes in the costs of fuel relative to the amount included in base rates. Effective July 1, 2016, with the implementation of the NMPRC Final Order, fuel and purchased power costs are no longer recovered through base rates but are recovered through the FPPCAC. The Company's request to reconcile its fuel and purchased power costs for the period January 1, 2013 through December 31, 2014 was approved in Case No. 15-00127-UT. New Mexico jurisdictional costs subject to prudence review are costs from January 1, 2015 through December 31, 2017 that total approximately $173.1 million. At December 31, 2017, the Company had a net fuel over-recovery balance of approximately $0.4 million in New Mexico. As required, the Company filed a request to continue use of its FPPCAC with the NMPRC on January 5, 2018 which was assigned NMPRC Case No. 18-00006-UT. 5 MW Holloman Air Force Base ("HAFB") Facility Certificate of Convenience and Necessity ("CCN"). On October 7, 2015, in NMPRC Case No. 15-00185-UT, the NMPRC issued a final order approving a CCN for a 5 MW solar power generation facility located on HAFB in the Company's service territory in New Mexico. The Company and HAFB negotiated a retail contract, which includes a power sales agreement for the facility, to replace the existing load retention agreement which was approved by final order issued October 5, 2016 in NMPRC Case No. 16-00224-UT. Construction of the solar generation facility is expected to be completed in the third quarter of 2018. New Mexico Efficient Use of Energy Recovery Factor. On July 1, 2016, the Company filed its annual application requesting approval of its 2017 Energy Efficiency and Load Management Plan and to establish energy efficiency cost recovery factors for 2017. In addition to projected energy efficiency costs for 2017, the Company requested approval of a $0.4 million incentive for 2017 energy efficiency programs in accordance with NMPRC rules. This case was assigned Case No. 16-00185-UT. On February 22, 2017, the NMPRC issued a Final Order approving the Company’s 2017 Energy Efficiency and Load Management Plan and authorizing recovery in 2017 of a base incentive of $0.4 million. The Company’s energy efficiency cost recovery factors were approved and effective in customer bills beginning on March 1, 2017. On July 1, 2016, the Company filed its 2015 Annual Report for Energy Efficiency Programs, which included an incentive for verified 2015 program performance of $0.3 million, which was approved in Case No. 13-00176-UT. The Company recorded the $0.3 million approved incentive in operating revenues in the first quarter of 2017. In addition, on June 30, 2017, the Company filed its 2016 Annual Report for Energy Efficiency Programs, which included an incentive for verified 2016 program performance of $0.4 million that was approved in Case No. 13-00176-UT. The Company recorded the $0.4 million approved incentive in operating revenues in the third quarter of 2017. Revolving Credit Facility, Issuance of Long-Term Debt, and Securities Financing. On October 7, 2015, the Company received approval in NMPRC Case No. 15-00280-UT to guarantee the issuance of up to $65.0 million of long-term debt by the Rio Grande Resources Trust ("RGRT") to finance future purchases of nuclear fuel and to refinance existing nuclear fuel debt obligations, which remains effective. On October 4, 2017, the Company received additional approval in NMPRC Case No. 17-00217-UT to amend and extend its Revolving Credit Facility ("RCF"), issue up to $350.0 million in long-term debt and to redeem and refinance the $63.5 million 2009 Series A 7.25% Pollution Control Bonds and the $37.1 million 2009 Series B 7.25% Pollution Control Bonds, which have optional redemptions beginning in 2019. The NMPRC approval to issue $350.0 million in long-term debt supersedes its prior approval. Other Required Approvals. The Company has obtained other required approvals for tariffs and other approvals as required by the New Mexico Public Utility Act and the NMPRC. Federal Regulatory Matters Revolving Credit Facility; Issuance of Long-Term Debt, Securities Financing, and Guarantee of Debt. On October 31, 2017, the FERC issued an order in Docket No. ES17-54-000 approving the Company’s filing to (i) amend and extend the RCF; (ii) issue up to $350.0 million in long-term debt; (iii) guarantee the issuance of up to $65.0 million of long-term debt by the RGRT; and (iv) redeem and refinance the $63.5 million 2009 Series A 7.25% Pollution Control Bonds and the $37.1 million 2009 Series B 7.25% Pollution Control Bonds, which have optional redemptions beginning in 2019. The order also approves the Company's request to continue to utilize the Company's existing RCF with the ability to amend and extend at a future date. The authorization is effective from November 15, 2017 through November 14, 2019 and supersedes prior FERC approvals. Other Required Approvals. The Company has obtained required approvals for rates, tariffs and other approvals as required by the FERC. United States Department of Energy ("DOE"). The DOE regulates the Company's exports of power to Mexico pursuant to a DOE grant of export authorization. In addition, the Company is the holder of two presidential permits issued by the DOE under which the Company constructed and operates border facilities crossing the United States/Mexico border. The DOE is authorized to assess operators of nuclear generating facilities a share of the costs of decommissioning the DOE's uranium enrichment facilities and for the ultimate costs of disposal of spent nuclear fuel. See Part II, Item 8, Financial Statements and Supplementary Data, Note E for discussion of spent fuel storage and disposal costs. Sales for Resale and Network Transmission Service to Rio Grande Electric Cooperative The Company provides firm capacity and associated energy to the Rio Grande Electric Cooperative ("RGEC") pursuant to an ongoing contract with a two-year notice to terminate provision. The Company also provides network integrated transmission service to the RGEC pursuant to the Company's Open Access Transmission Tariff ("OATT"). The contract includes a formula-based rate that is updated annually to recover non-fuel generation costs and a fuel adjustment clause designed to recover all eligible fuel and purchased power costs allocable to the RGEC. The Company's service to RGEC is regulated by FERC.

Regulation

General

The rates and services of the Company are regulated by incorporated municipalities in Texas, the PUCT, the NMPRC and the FERC. Municipal orders, ordinances and other agreements regarding rates and services adopted by Texas municipalities are subject to review and approval by the PUCT. The FERC has jurisdiction over the Company's wholesale (sales for resale) transactions, transmission service and compliance with federally-mandated reliability standards. The decisions of the PUCT, the NMPRC and the FERC are subject to judicial review.

Texas Regulatory Matters

2015 Texas Retail Rate Case Filing. On August 10, 2015, the Company filed with the City of El Paso, other municipalities incorporated in its Texas service territory, and the PUCT in Docket No. 44941, a request for an annual increase in non-fuel base revenues ("2015 Texas Retail Rate Case").

On July 21, 2016, the parties to PUCT Docket No. 44941 filed the Joint Motion to Implement Uncontested Amended and Restated Stipulation and Agreement which was unopposed by the parties (the "2016 Unopposed Settlement"). On August 25, 2016, the PUCT approved the 2016 Unopposed Settlement and issued the 2016 PUCT Final Order, as proposed. The 2016 PUCT Final Order provided for: (i) an annual non-fuel base rate increase, lower annual depreciation expense, a revised return on equity for AFUDC purposes, and the inclusion of substantially all new plant in service in rate base; (ii) an additional annual non-fuel base rate increase of $3.7 million related to Four Corners Generating Station ("Four Corners") costs, which was collected through a surcharge that terminated on July 11, 2017; (iii) removing the separate rate treatment for residential customers with solar systems that the Company had proposed in its August 10, 2015 filing; (iv) allowing the Company to recover $3.1 million in rate case expenses through a separate surcharge; and (v) allowing the Company to recover revenues associated with the relate back of rates to consumption on and after January 12, 2016 through March 31, 2016 through a separate surcharge.

Interim rates associated with the annual non-fuel base rate increase became effective on April 1, 2016. The additional surcharges associated with the incremental Four Corners costs, rate case expenses and the relate back of rates to consumption on and after January 12, 2016 through March 31, 2016 were implemented on October 1, 2016.

For financial reporting purposes, the Company deferred any recognition of the Company's request in its 2015 Texas Retail Rate Case until it received the 2016 PUCT Final Order on August 25, 2016. Accordingly, it reported in the third quarter of 2016 the cumulative effect of the 2016 PUCT Final Order, which related back to January 12, 2016.

2017 Texas Retail Rate Case Filing. On February 13, 2017, the Company filed with the City of El Paso, other municipalities incorporated in the Company's Texas service territory and the PUCT in Docket No. 46831, a request for an increase in non-fuel base revenues ("2017 Texas Retail Rate Case"). On November 2, 2017, the Company filed the Joint Motion to Implement Uncontested Stipulation and Agreement with the Administrative Law Judges for the 2017 Texas Retail Rate Case.

On December 18, 2017, the PUCT issued its final order in the Company's rate case pending in Docket No. 46831 ("2017 PUCT Final Order"), which provides, among other things, for the following: (i) an annual non-fuel base rate increase of $14.5 million; (ii) a return on equity of 9.65%; (iii) all new plant in service as filed in the Company's rate filing package was prudent and used and useful and therefore is included in rate base; (iv) recovery of the costs of decommissioning Four Corners in the amount of $5.5 million over a seven year period beginning August 1, 2017; (v) the Company to recover reasonable rate case expenses of approximately $3.4 million through a separate surcharge over a three year period; and (vi) a requirement that the Company file a refund tariff if the federal statutory income tax rate, as it relates to the Company, is decreased before the Company files its next rate case. The 2017 PUCT Final Order also establishes baseline revenue requirements for recovery of future transmission and distribution investment costs, and includes a minimum monthly bill of $30.00 for new residential customers with distributed generation, such as private rooftop solar. Additionally, the 2017 PUCT Final Order allows for the annual recovery of $2.1 million of nuclear decommissioning funding and establishes annual depreciation expense that is approximately $1.9 million lower than the annual amount requested by the Company in its initial filing. Finally, the 2017 PUCT Final Order allows for the Company to recover revenues associated with the relate back of rates to consumption on and after July 18, 2017 through a separate surcharge.

New base rates, including additional surcharges associated with rate case expenses and the relate back of rates to consumption on and after July 18, 2017 through December 31, 2017 were implemented in January 2018.

For financial reporting purposes, the Company deferred any recognition of the Company's request in its 2017 Texas Retail Rate Case until it received the 2017 PUCT Final Order on December 18, 2017. Accordingly, it reported in the fourth quarter of 2017 the cumulative effect of the 2017 PUCT Final Order, which related back to July 18, 2017.

The 2017 PUCT Final Order requires the Company to file a refund tariff if the federal statutory income tax rate, as it relates to the Company, is decreased before the Company files its next rate case. Following the enactment of the TCJA on December 22, 2017, and in compliance with the 2017 PUCT Final Order, the Company will reduce the recognition of Texas jurisdictional revenues beginning January 1, 2018, to approximate the tax savings resulting from the TCJA and will file a refund tariff which the Company will ask to be implemented in the first half of 2018. The refund tariff is expected to be reflected in rates over a period of a year and will be updated annually until new base rates are implemented pursuant to the Company's next rate case filing. See Part II, Item 8, Financial Statements and Supplementary Data, Note J for further details.

Energy Efficiency Cost Recovery Factor. On May 1, 2017, the Company filed its annual application, which was assigned PUCT Docket No. 47125, to establish its energy efficiency cost recovery factor ("EECRF") for 2018. In addition to projected energy efficiency costs for 2018 and a true-up to prior year actual costs, the Company requested approval of an incentive bonus for the 2016 energy efficiency program results in accordance with PUCT rules. Interim rates were approved effective January 1, 2018. The Company, the staff of the PUCT, and the City of El Paso reached an agreement that includes an incentive bonus of $0.8 million. The agreement was filed on January 25, 2018, and was approved by the PUCT on February 15, 2018.

Fuel and Purchased Power Costs. The Company's actual fuel costs, including purchased power energy costs, are recovered from customers through a fixed fuel factor. The PUCT has adopted a fuel cost recovery rule (the "Texas Fuel Rule") that allows the Company to seek periodic adjustments to its fixed fuel factor. The Company can seek to revise its fixed fuel factor based upon the approved formula at least four months after its last revision except in the month of December. The Texas Fuel Rule requires the Company to request to refund fuel costs in any month when the over-recovery balance exceeds a threshold material amount and it expects fuel costs to continue to be materially over-recovered. The Texas Fuel Rule also permits the Company to seek to surcharge fuel under-recoveries in any month the balance exceeds a threshold material amount and it expects fuel cost recovery to continue to be materially under-recovered. Fuel over and under-recoveries are considered material when they exceed 4% of the previous twelve months' fuel costs. All such fuel revenue and expense activities are subject to periodic final review by the PUCT in fuel reconciliation proceedings.

On November 30, 2016, the Company filed a request, which was assigned PUCT Docket No. 46610, to increase its fixed fuel factor by approximately 28.8% to reflect increased fuel expenses primarily related to an increase in the price of natural gas used to generate power. The increase in the fixed fuel factor was effective on an interim basis January 1, 2017 and approved by the PUCT on January 10, 2017. As of September 30, 2017, the Company had over-recovered fuel costs in the amount of $1.1 million for the Texas jurisdiction. On October 13, 2017, the Company filed a request, which was assigned PUCT Docket No. 47692, to decrease the Texas fixed fuel factor by approximately 19% to reflect decreased fuel expenses primarily related to a decrease in the price of natural gas used to generate power. The decrease in the Texas fixed fuel factor became effective beginning with the November 2017 billing month and will continue thereafter until changed by the PUCT. At December 31, 2017, the Company had a net fuel over-recovery balance of approximately $5.8 million in Texas.

Fuel Reconciliation Proceeding. On September 27, 2016, the Company filed an application with the PUCT, designated as PUCT Docket No. 46308, to reconcile $436.6 million of Texas fuel and purchased power expenses incurred during the period of April 1, 2013 through March 31, 2016. On June 29, 2017, the PUCT approved a settlement in this proceeding. The settlement provides for the reconciliation of fuel and purchased power costs incurred from April 1, 2013 through March 31, 2016. Additionally, the settlement modifies and tightens the Palo Verde performance rewards measurement bands beginning with the 2018 performance period. The financial results for the twelve months ended December 31, 2017 include a $5.0 million, pre-tax increase to income reflecting the settlement of the Texas fuel reconciliation proceeding. This amount represents Palo Verde performance rewards associated with the 2013 to 2015 performance periods net of disallowed fuel and purchased power costs as approved in the settlement. Texas jurisdictional fuel and purchased power costs subject to prudence review are costs from April 1, 2016 through December 31, 2017 that total approximately $250.9 million.

Community Solar. On June 8, 2015, the Company filed a petition with the PUCT to initiate a community solar program that includes the construction and ownership of a 3 MW solar photovoltaic system located at the Company's Montana Power Station ("MPS"). Participation is on a voluntary basis, and customers contract for a set capacity (kW) amount and receive all energy produced. This case was assigned PUCT Docket No. 44800. The Company filed a settlement agreement among all parties on July 1, 2016 approving the program, and the PUCT approved the settlement agreement and program on September 1, 2016. On April 19, 2017, the Company announced that the entire 3 MW program was fully subscribed by approximately 1,500 Texas customers. The Community Solar facility began commercial operation on May 31, 2017.

Four Corners Generating Station. On February 17, 2015, the Company and Arizona Public Service Company ("APS") entered into an asset purchase agreement (the "Purchase and Sale Agreement") providing for the sale of the Company's interest in Four Corners to APS. The sale of the Company's interest in Four Corners closed on July 6, 2016. See Part II, Item 8, Financial Statements and Supplementary Data, Note E for further details on the sale of Four Corners.

On June 10, 2015, the Company filed an application in Texas requesting reasonableness and public interest findings and certain rate and accounting findings related to the Purchase and Sale Agreement. This case was assigned PUCT Docket No. 44805. Subsequent to the filing of the application, the case was subject to numerous procedural matters, including a March 23, 2016 order in which the PUCT determined not to dismiss the reasonableness and public interest issues in this docket but to consider the requested rate and accounting findings, including coal mine reclamation costs, in a rate case proceeding. On September 1, 2016, a motion by parties in the proceeding to suspend the procedural schedule in order to pursue settlement was approved. On March 3, 2017, the Company filed a Joint Motion to Implement Stipulation and Agreement (the "Stipulation and Agreement"), and PUCT Staff filed its recommendation that the Company’s disposition of its interest in Four Corners was reasonable and consistent with the public interest. Additionally, the signatories of the Stipulation and Agreement agreed to support the recovery of the Company's Four Corners decommissioning costs in the 2017 Texas Retail Rate Case. A final order approving the Stipulation and Agreement was adopted by the PUCT on March 30, 2017. The approval to recover Four Corners decommissioning costs was included in the 2017 PUCT Final Order.

Other Required Approvals. The Company has obtained other required approvals for tariffs and other approvals required by the Texas Public Utility Regulatory Act and the PUCT.

New Mexico Regulatory Matters

2015 New Mexico Rate Case Filing. On May 11, 2015, the Company filed a request with the NMPRC, in Case No. 15-00127-UT, for an annual increase in non-fuel base rates. On June 8, 2016, the NMPRC issued the NMPRC Final Order which approved an annual increase in non-fuel base rates of approximately $0.6 million, an increase of approximately $0.5 million in other service fees and a decrease in the Company's allowed return on equity to 9.48%. The NMPRC Final Order concluded that all of the Company's new plant in service was reasonable and necessary and therefore would be recoverable in rates. The Company's rates were approved by the NMPRC effective July 1, 2016 and implemented at such time.

Future New Mexico Rate Case Filing. NMPRC Case No. 15-00109-UT required the Company to make a rate filing in New Mexico in the second quarter of 2017 using a historical test year ended December 31, 2016. On March 24, 2017, the Company, NMPRC Utility Division Staff and the New Mexico Attorney General filed a Joint Motion to Modify Filing Date Stated in Final Order requesting that the rate filing date be changed to no later than July 31, 2019, using the appropriate historical test year period. The joint request was approved by the NMPRC on April 12, 2017. The NMPRC has initiated an investigation into the impact of the TCJA on utility customers that may require earlier action by the Company. The Company is evaluating possible approaches to begin providing a refund credit for the TCJA income tax rate decrease to New Mexico customers.

Fuel and Purchased Power Costs.Historically, fuel and purchased power costs were recovered through base rates and a Fuel and Purchased Power Cost Adjustment Clause (the "FPPCAC") that accounts for changes in the costs of fuel relative to the amount included in base rates. Effective July 1, 2016, with the implementation of the NMPRC Final Order, fuel and purchased power costs are no longer recovered through base rates but are recovered through the FPPCAC. The Company's request to reconcile its fuel and purchased power costs for the period January 1, 2013 through December 31, 2014 was approved in Case No. 15-00127-UT. New Mexico jurisdictional costs subject to prudence review are costs from January 1, 2015 through December 31, 2017 that total approximately $173.1 million. At December 31, 2017, the Company had a net fuel over-recovery balance of approximately $0.4 million in New Mexico. As required, the Company filed a request to continue use of its FPPCAC with the NMPRC on January 5, 2018 which was assigned NMPRC Case No. 18-00006-UT.

5 MW Holloman Air Force Base ("HAFB") Facility Certificate of Convenience and Necessity ("CCN"). On October 7, 2015, in NMPRC Case No. 15-00185-UT, the NMPRC issued a final order approving a CCN for a 5 MW solar power generation facility located on HAFB in the Company's service territory in New Mexico. The Company and HAFB negotiated a retail contract, which includes a power sales agreement for the facility, to replace the existing load retention agreement which was approved by final order issued October 5, 2016 in NMPRC Case No. 16-00224-UT. Construction of the solar generation facility is expected to be completed in the third quarter of 2018.

New Mexico Efficient Use of Energy Recovery Factor. On July 1, 2016, the Company filed its annual application requesting approval of its 2017 Energy Efficiency and Load Management Plan and to establish energy efficiency cost recovery factors for 2017. In addition to projected energy efficiency costs for 2017, the Company requested approval of a $0.4 million incentive for 2017 energy efficiency programs in accordance with NMPRC rules. This case was assigned Case No. 16-00185-UT. On February 22, 2017, the NMPRC issued a Final Order approving the Company’s 2017 Energy Efficiency and Load Management Plan and authorizing recovery in 2017 of a base incentive of $0.4 million. The Company’s energy efficiency cost recovery factors were approved and effective in customer bills beginning on March 1, 2017.

On July 1, 2016, the Company filed its 2015 Annual Report for Energy Efficiency Programs, which included an incentive for verified 2015 program performance of $0.3 million, which was approved in Case No. 13-00176-UT. The Company recorded the $0.3 million approved incentive in operating revenues in the first quarter of 2017. In addition, on June 30, 2017, the Company filed its 2016 Annual Report for Energy Efficiency Programs, which included an incentive for verified 2016 program performance of $0.4 million that was approved in Case No. 13-00176-UT. The Company recorded the $0.4 million approved incentive in operating revenues in the third quarter of 2017.

Revolving Credit Facility, Issuance of Long-Term Debt, and Securities Financing. On October 7, 2015, the Company received approval in NMPRC Case No. 15-00280-UT to guarantee the issuance of up to $65.0 million of long-term debt by the Rio Grande Resources Trust ("RGRT") to finance future purchases of nuclear fuel and to refinance existing nuclear fuel debt obligations, which remains effective. On October 4, 2017, the Company received additional approval in NMPRC Case No. 17-00217-UT to amend and extend its Revolving Credit Facility ("RCF"), issue up to $350.0 million in long-term debt and to redeem and refinance the $63.5 million 2009 Series A 7.25% Pollution Control Bonds and the $37.1 million 2009 Series B 7.25% Pollution Control Bonds, which have optional redemptions beginning in 2019. The NMPRC approval to issue $350.0 million in long-term debt supersedes its prior approval.

Other Required Approvals. The Company has obtained other required approvals for tariffs and other approvals as required by the New Mexico Public Utility Act and the NMPRC.

Federal Regulatory Matters

Revolving Credit Facility; Issuance of Long-Term Debt, Securities Financing, and Guarantee of Debt. On October 31, 2017, the FERC issued an order in Docket No. ES17-54-000 approving the Company’s filing to (i) amend and extend the RCF; (ii) issue up to $350.0 million in long-term debt; (iii) guarantee the issuance of up to $65.0 million of long-term debt by the RGRT; and (iv) redeem and refinance the $63.5 million 2009 Series A 7.25% Pollution Control Bonds and the $37.1 million 2009 Series B 7.25% Pollution Control Bonds, which have optional redemptions beginning in 2019. The order also approves the Company's request to continue to utilize the Company's existing RCF with the ability to amend and extend at a future date. The authorization is effective from November 15, 2017 through November 14, 2019 and supersedes prior FERC approvals.

Other Required Approvals. The Company has obtained required approvals for rates, tariffs and other approvals as required by the FERC.

United States Department of Energy ("DOE"). The DOE regulates the Company's exports of power to Mexico pursuant to a DOE grant of export authorization. In addition, the Company is the holder of two presidential permits issued by the DOE under which the Company constructed and operates border facilities crossing the United States/Mexico border.

The DOE is authorized to assess operators of nuclear generating facilities a share of the costs of decommissioning the DOE's uranium enrichment facilities and for the ultimate costs of disposal of spent nuclear fuel. See Part II, Item 8, Financial Statements and Supplementary Data, Note E for discussion of spent fuel storage and disposal costs.

Sales for Resale and Network Transmission Service to Rio Grande Electric Cooperative

The Company provides firm capacity and associated energy to the Rio Grande Electric Cooperative ("RGEC") pursuant to an ongoing contract with a two-year notice to terminate provision. The Company also provides network integrated transmission service to the RGEC pursuant to the Company's Open Access Transmission Tariff ("OATT"). The contract includes a formula-based rate that is updated annually to recover non-fuel generation costs and a fuel adjustment clause designed to recover all eligible fuel and purchased power costs allocable to the RGEC. The Company's service to RGEC is regulated by FERC.