Income Taxes	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes On December 22, 2017, comprehensive changes in United States federal income taxes were enacted through legislation commonly known as the Tax Cuts and Jobs Act (the “Tax Act”). The Tax Act makes many significant modifications to the tax laws, including reducing the federal corporate income tax rate from 35% to 21% effective January 1, 2018. The Tax Act also eliminates federal bonus depreciation for utilities effective September 28, 2017 and, effective January 1, 2018, limits interest deductibility for non-utility businesses and limits the deductibility of certain officer compensation. Although most of the provisions of the Tax Act are not effective until 2018, GAAP required that some effects be recognized in 2017. Under the asset and liability method of accounting for income taxes used by the Company, deferred tax assets and liabilities are recognized for the future tax consequences of temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. The deferred tax assets and liabilities are measured using the enacted tax rates expected to apply to taxable income in the years in which the temporary differences are expected to reverse. At the date of enactment of the Tax Act, the Company had net deferred tax liabilities for its regulated activities and net deferred tax assets for non-regulated activities. As a result of the change in the federal corporate income tax rate, the Company re-measured and adjusted its deferred tax assets and liabilities as of December 31, 2017. The portion of that adjustment not related to PNM’s and TNMP’s regulated activities was recorded as a reduction in net deferred tax assets and an increase in income tax expense. The portion related to PNM’s and TNMP’s regulated activities was recorded as a reduction in net deferred tax liabilities and an increase in regulatory liabilities, based on the assumption that PNM and TNMP will be required to return the benefit to ratepayers over time. PNM’s NM 2016 Rate Case (Note 12) reflects that assumption by including an amortization of the estimated benefit of the reduction in existing deferred federal corporate income taxes as a reduction to customer rates over a twenty-one year period beginning in 2018. On January 25, 2018, the PUCT issued an order requiring Texas utilities, including TNMP, to begin recording regulatory liabilities for the effects of the Tax Act with the stated purpose of reflecting those effects in the utility bills of Texas ratepayers. During the three and nine months ended September 30, 2018, TNMP reduced revenue and recorded a regulatory liability of $1.5 million and $4.2 million in accordance with the PUCT’s order. The TNMP 2018 Rate Case filed on May 30, 2018, and related settlement agreement include a reduction in customer rates to reflect the impacts of the Tax Act, including amortization of the regulatory liability related to the 2017 re-measurement of deferred tax liabilities and to reduce the federal corporate income tax rate to 21% (Note 12). In December 2017, the SEC issued Staff Accounting Bulletin No. 118, which provides guidance to address the application of GAAP to reflect the Tax Act in circumstances where all information and analysis of the Tax Act is not yet available or complete. This bulletin provides for up to a one-year period in which to complete the required analyses and accounting for the impacts of the Tax Act. The Company believes it made reasonable estimates of the effects of the Tax Act and reflected the impacts in the Consolidated Financial Statements included in the 2017 Annual Reports on Form 10-K. However, the reported effects on the Company’s deferred tax assets and liabilities, regulatory assets and liabilities, and income tax expense are provisional and it is possible that changes to United States Treasury regulations, IRS interpretations of the provisions of the Tax Act, actions by the NMPRC, PUCT, and FERC, or the Company’s further analysis of historical records could cause these estimates to change. Through September 30, 2018, no significant adjustments to the impacts reflected in the 2017 Consolidated Financial Statements included in the 2017 Annual Reports on Form 10-K have been identified. In August 2018, the IRS issued new guidance clarifying the deductibility of executive compensation under Section 162(m) of the Internal Revenue Code as amended by the Tax Act. In addition, the IRS issued proposed regulations interpreting Tax Act amendments to depreciation provisions of the Internal Revenue Code which would allow the Company to claim a bonus depreciation deduction on certain construction projects placed in service during the fourth quarter of 2017. The Company is currently evaluating the IRS’s new guidance and proposed regulations and anticipates it will record adjustments, if any, prior to December 31, 2018. In 2013, New Mexico House Bill 641 reduced the New Mexico corporate income tax rate from 7.6% to 5.9%. The rate reduction is being phased-in from 2014 to 2018. In accordance with GAAP, PNMR and PNM adjusted accumulated deferred income taxes to reflect the tax rate at which the balances are expected to reverse during the period that includes the date of enactment, which was in the year ended December 31, 2013. At that time, the portion of the adjustment related to PNM’s regulated activities was recorded as a reduction in deferred tax liabilities and an increase in a regulatory liability, based on the assumption that PNM would be required to return the benefit to customers over time. PNM’s NM 2016 Rate Case (Note 12) reflects the benefit of the lower New Mexico corporate income tax rate being returned to customers over a three-year period beginning February 1, 2018. In addition, the portion of the adjustment that was not related to PNM’s regulated activities was recorded as a reduction in deferred tax assets and an increase in income tax expense. Changes in the estimated timing of reversals of deferred tax assets and liabilities resulted in refinements of the impacts of this change in tax rates being recorded periodically through December 31, 2017, at which time the impacts of the rate reduction were fully phased-in. In the three months ended March 31, 2017, PNM’s regulatory liability was reduced by $4.8 million, which increased deferred tax liabilities. Deferred tax assets not related to PNM’s regulatory activities were reduced by $0.1 million in the three months ended March 31, 2017, increasing income tax expense by less than $0.1 million for PNM and $0.1 million for the Corporate and Other segment. As required under GAAP, the Company makes an estimate of its anticipated effective tax rate for the year as of the end of each quarterly period within its fiscal year. In interim periods, income tax expense is calculated by applying the anticipated annual effective tax rate to year-to-date earnings before income taxes, which includes the earnings attributable to the Valencia non-controlling interest. GAAP also provides that certain unusual or infrequently occurring items, including excess tax benefits related to stock awards, be excluded from the estimated annual effective tax rate calculation. At September 30, 2018, PNMR, PNM, and TNMP estimated their effective income tax rates for the year ended December 31, 2018 would be 12.35%, 9.51%, and 22.78%. These rates reflect the reduced federal corporate income tax rate of 21%, which rates are adjusted to reflect permanent differences between earnings determined in accordance with GAAP and taxable income, as well as state income taxes. The primary permanent difference is the reduction in income tax expense resulting from the amortization of excess deferred federal and state income taxes ordered by the NMPRC in PNM’s NM 2016 Rate Case. During the three and nine months ended September 30, 2018, income tax expense calculated by applying the expected annual effective income tax rate to earnings before income taxes was further reduced by excess tax benefits related to stock awards of zero and $1.4 million for PNMR, zero and $1.0 million for PNM, and less than $0.1 million and $0.4 million for TNMP.

Income Taxes

On December 22, 2017, comprehensive changes in United States federal income taxes were enacted through legislation commonly known as the Tax Cuts and Jobs Act (the “Tax Act”). The Tax Act makes many significant modifications to the tax laws, including reducing the federal corporate income tax rate from 35% to 21% effective January 1, 2018. The Tax Act also eliminates federal bonus depreciation for utilities effective September 28, 2017 and, effective January 1, 2018, limits interest deductibility for non-utility businesses and limits the deductibility of certain officer compensation.

Although most of the provisions of the Tax Act are not effective until 2018, GAAP required that some effects be recognized in 2017. Under the asset and liability method of accounting for income taxes used by the Company, deferred tax assets and liabilities are recognized for the future tax consequences of temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. The deferred tax assets and liabilities are measured using the enacted tax rates expected to apply to taxable income in the years in which the temporary differences are expected to reverse. At the date of enactment of the Tax Act, the Company had net deferred tax liabilities for its regulated activities and net deferred tax assets for non-regulated activities. As a result of the change in the federal corporate income tax rate, the Company re-measured and adjusted its deferred tax assets and liabilities as of December 31, 2017. The portion of that adjustment not related to PNM’s and TNMP’s regulated activities was recorded as a reduction in net deferred tax assets and an increase in income tax expense. The portion related to PNM’s and TNMP’s regulated activities was recorded as a reduction in net deferred tax liabilities and an increase in regulatory liabilities, based on the assumption that PNM and TNMP will be required to return the benefit to ratepayers over time. PNM’s NM 2016 Rate Case (Note 12) reflects that assumption by including an amortization of the estimated benefit of the reduction in existing deferred federal corporate income taxes as a reduction to customer rates over a twenty-one year period beginning in 2018. On January 25, 2018, the PUCT issued an order requiring Texas utilities, including TNMP, to begin recording regulatory liabilities for the effects of the Tax Act with the stated purpose of reflecting those effects in the utility bills of Texas ratepayers. During the three and nine months ended September 30, 2018, TNMP reduced revenue and recorded a regulatory liability of $1.5 million and $4.2 million in accordance with the PUCT’s order. The TNMP 2018 Rate Case filed on May 30, 2018, and related settlement agreement include a reduction in customer rates to reflect the impacts of the Tax Act, including amortization of the regulatory liability related to the 2017 re-measurement of deferred tax liabilities and to reduce the federal corporate income tax rate to 21% (Note 12).

In December 2017, the SEC issued Staff Accounting Bulletin No. 118, which provides guidance to address the application of GAAP to reflect the Tax Act in circumstances where all information and analysis of the Tax Act is not yet available or complete. This bulletin provides for up to a one-year period in which to complete the required analyses and accounting for the impacts of the Tax Act. The Company believes it made reasonable estimates of the effects of the Tax Act and reflected the impacts in the Consolidated Financial Statements included in the 2017 Annual Reports on Form 10-K. However, the reported effects on the Company’s deferred tax assets and liabilities, regulatory assets and liabilities, and income tax expense are provisional and it is possible that changes to United States Treasury regulations, IRS interpretations of the provisions of the Tax Act, actions by the NMPRC, PUCT, and FERC, or the Company’s further analysis of historical records could cause these estimates to change. Through September 30, 2018, no significant adjustments to the impacts reflected in the 2017 Consolidated Financial Statements included in the 2017 Annual Reports on Form 10-K have been identified. In August 2018, the IRS issued new guidance clarifying the deductibility of executive compensation under Section 162(m) of the Internal Revenue Code as amended by the Tax Act. In addition, the IRS issued proposed regulations interpreting Tax Act amendments to depreciation provisions of the Internal Revenue Code which would allow the Company to claim a bonus depreciation deduction on certain construction projects placed in service during the fourth quarter of 2017. The Company is currently evaluating the IRS’s new guidance and proposed regulations and anticipates it will record adjustments, if any, prior to December 31, 2018.

In 2013, New Mexico House Bill 641 reduced the New Mexico corporate income tax rate from 7.6% to 5.9%. The rate reduction is being phased-in from 2014 to 2018. In accordance with GAAP, PNMR and PNM adjusted accumulated deferred income taxes to reflect the tax rate at which the balances are expected to reverse during the period that includes the date of enactment, which was in the year ended December 31, 2013. At that time, the portion of the adjustment related to PNM’s regulated activities was recorded as a reduction in deferred tax liabilities and an increase in a regulatory liability, based on the assumption that PNM would be required to return the benefit to customers over time. PNM’s NM 2016 Rate Case (Note 12) reflects the benefit of the lower New Mexico corporate income tax rate being returned to customers over a three-year period beginning February 1, 2018. In addition, the portion of the adjustment that was not related to PNM’s regulated activities was recorded as a reduction in deferred tax assets and an increase in income tax expense. Changes in the estimated timing of reversals of deferred tax assets and liabilities resulted in refinements of the impacts of this change in tax rates being recorded periodically through December 31, 2017, at which time the impacts of the rate reduction were fully phased-in. In the three months ended March 31, 2017, PNM’s regulatory liability was reduced by $4.8 million, which increased deferred tax liabilities. Deferred tax assets not related to PNM’s regulatory activities were reduced by $0.1 million in the three months ended March 31, 2017, increasing income tax expense by less than $0.1 million for PNM and $0.1 million for the Corporate and Other segment.

As required under GAAP, the Company makes an estimate of its anticipated effective tax rate for the year as of the end of each quarterly period within its fiscal year. In interim periods, income tax expense is calculated by applying the anticipated annual effective tax rate to year-to-date earnings before income taxes, which includes the earnings attributable to the Valencia non-controlling interest. GAAP also provides that certain unusual or infrequently occurring items, including excess tax benefits related to stock awards, be excluded from the estimated annual effective tax rate calculation. At September 30, 2018, PNMR, PNM, and TNMP estimated their effective income tax rates for the year ended December 31, 2018 would be 12.35%, 9.51%, and 22.78%. These rates reflect the reduced federal corporate income tax rate of 21%, which rates are adjusted to reflect permanent differences between earnings determined in accordance with GAAP and taxable income, as well as state income taxes. The primary permanent difference is the reduction in income tax expense resulting from the amortization of excess deferred federal and state income taxes ordered by the NMPRC in PNM’s NM 2016 Rate Case. During the three and nine months ended September 30, 2018, income tax expense calculated by applying the expected annual effective income tax rate to earnings before income taxes was further reduced by excess tax benefits related to stock awards of zero and $1.4 million for PNMR, zero and $1.0 million for PNM, and less than $0.1 million and $0.4 million for TNMP.