Investments, Debt and Equity Securities [Abstract] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Debt Securities, Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Federal Agency Mortgage Backed Securities Collateralized Mortgage Backed Securities [Member] U.S. Government Bonds US Treasury and Government [Member] Municipal Obligations Municipal Bonds [Member] Corporate Obligations Corporate Debt Securities [Member] Schedule of Available-for-sale Securities [Line Items] Total Available-for-sale Securities, Debt Securities 2018 Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value 2019 through 2022 Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value 2023 through 2027 Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value 2028 and Beyond Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value Adjustments for New Accounting Pronouncements [Abstract] Adjustments for New Accounting Pronouncements [Abstract] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Net periodic benefit cost previously reported in operating income (including non-service cost components) Scenario, Previously Reported [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2017-07 Accounting Standards Update 2017-07 [Member] Accounting Standards Update 2017-07 Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Operations and maintenance Utilities Operating Expense, Maintenance and Operations Investment and interest income, net Interest And Other Investment Income Loss This item represents investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums, respectively, net of related investment expenses. This item includes realized or unrealized gains or losses on the sale or holding of investments in debt and equity securities required to be included in earnings for the period or for other than temporary losses related to investments in debt and equity securities which are included in realized losses in the period recognized, and does not include investment income from real or personal property, such as rental income. Also, includes other interest income from operating activities. Miscellaneous non-operating income Other Nonoperating Income Miscellaneous non-operating deductions Other Nonoperating Expense Other interest Interest Expense, Other Service Cost Component Defined Benefit Plan, Service Cost Revenue Recognition [Abstract] Revenue Recognition for Alternative Revenue Programs, Policy Revenue Recognition for Alternative Revenue Programs, Policy [Policy Text Block] Statement of Financial Position [Abstract] Statement [Table] Statement [Table] Statement, Scenario [Axis] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Statement [Line Items] Statement [Line Items] Nuclear fuel; fuel in process Nuclear fuel; fuel in process Nuclear fuel, Fuel in process Allowance for Doubtful Accounts Receivable, Current Allowance for Doubtful Accounts Receivable, Current Common Stock, Par or Stated Value Per Share Common Stock, Par or Stated Value Per Share Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares Issued Common Stock, Shares, Issued Common stock, Restricted shares Common stock, restricted shares The total number of unvested common shares that contain restrictions. Treasury Stock, Shares Treasury Stock, Shares Palo Verde [Abstract] Palo Verde [Abstract] Palo Verde Property, Plant and Equipment Disclosure [Text Block] Supplemental Cash Flow Information [Abstract] Cash Flow, Supplemental Disclosures Cash Flow, Supplemental Disclosures [Text Block] Other Comprehensive Income (Loss), Net of Tax [Abstract] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Schedule of Accumulated Other Comprehensive Income (Loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Reclassification out of Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Payments to fund Decommissioning Fund Contribution to decommissioning trust fund for the purchase of investments Contribution to decommissioning trust funds for the purchase of investments. Fair Value Disclosures [Abstract] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping, Disclosure Item Amounts (Axis) Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Carrying (Reported) Amount, Fair Value Disclosure Reported Value Measurement [Member] Estimate of Fair Value Measurement Estimate of Fair Value Measurement [Member] Fair Value by Liability Class [Axis] Liability Class [Axis] Fair Value, Liabilities, Measured on Recurring Basis, Unobservable Input Reconciliation, by Liability Class [Domain] Fair Value by Liability Class [Domain] Pollution Control Bonds Pollution Control Bonds [Member] Pollution Control Bonds [Member] Senior Notes Senior Notes [Member] RGRT Senior Notes Rgrt Senior Notes [Member] Rio Grande Resources Trust II Senior Notes [Member] Revolving Credit Facility Revolving Credit Facility [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Debt Instrument, Fair Value Disclosure Debt Instrument, Fair Value Disclosure Investments [Abstract] Investment [Table] Investment [Axis] Investment [Axis] Investment [Domain] Investment [Domain] Investment [Line Items] Investment [Line Items] Proceeds from sales or maturities of available-for-sale securities Proceeds from Sale and Maturity of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities, Debt Proceeds from Sale of Available-for-sale Securities, Debt Gross realized gains included in pre-tax income Available-for-sale Securities, Gross Realized Gains Gross realized losses included in pre-tax income Available-for-sale Securities, Gross Realized Losses Gross unrealized losses included in pre-tax income Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities Net gains included in pre-tax income Marketable Securities, Realized Gain (Loss) Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant [Abstract] Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant [Abstract] Schedule of Property, Plant and Equipment and Finite-Lived Intangible Assets [Table] Schedule of Property, Plant and Equipment and Finite-Lived Intangible Assets [Table] Schedule of Property, Plant and Equipment and Finite-Lived Intangible Assets [Table] Litigation Information [Axis] Litigation Information [Axis] Litigation Information [Axis] Litigation Information [Domain] Litigation Information [Domain] Litigation Information [Domain] Suit Against Department of Energy for Spent Fuel Suit Against Department of Energy for Spent Fuel [Member] Suit Against Department of Energy for Spent Fuel [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Palo Verde Generating Station Jointly Owned Nuclear Power Plant [Member] Jointly Owned Utility Plant Interests [Line Items] Jointly Owned Utility Plant Interests [Line Items] Settlement Agreement, Counterparty's Name Loss Contingency, Settlement Agreement, Counterparty's Name Litigation, Period Covered Loss Contingency, Period of Occurrence Loss Contingency, Settlement Agreement, Date Loss Contingency, Settlement Agreement, Date Refund passed through to customers Increase (Decrease) in Deferred Fuel Costs Proceeds from Legal Settlements Proceeds from Legal Settlements Regulated Operations [Abstract] Regulation Public Utilities Disclosure [Text Block] Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Accumulated Defined Benefit Plans Adjustment Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Accumulated Net Unrealized Investment Gain (Loss) Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss), Net of Tax AOCI Attributable to Parent [Member] AOCI Attributable to Parent AOCI Attributable to Parent, Net of Tax [Roll Forward] AOCI Attributable to Parent, Net of Tax [Roll Forward] Accumulated Other Comprehensive Income (Loss), beginning balance Accumulated Other Comprehensive Income (Loss), Net of Tax New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets Other Comprehensive Income (Loss) before Reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Accumulated Other Comprehensive Income (Loss), ending balance Interest on long-term debt and borrowings under the revolving credit facility Interest Paid Proceeds from income tax refunds Proceeds from Income Tax Refunds Changes in accrued plant additions Increase Decrease In Capital Expenditures Incurred But Not Yet Paid (Increase) Decrease in capital expenditures incurred but not yet paid Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Restricted Stock [Member] Performance Shares Performance Shares [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Common stock issued Stock Issued Supplemental Cash Flow Elements [Abstract] Supplemental Cash Flow Elements [Abstract] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Domestic Equity Securities Domestic Equity Securities [Member] Domestic Equity Securities [Member] Debt Securities Debt Securities [Member] Municipal Obligations Corporate Obligations Equity Securities Equity Securities [Member] Financial Instruments and Investments [Line Items] Financial Instruments and Investments [Line Items] Marketable Securities Fair Value Less than 12 Months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value Marketable Securities Unrealized Losses Less Than 12 Months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Marketable Securities Fair Value 12 Months or Longer Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Marketable Securities Unrealized Losses 12 Months or Longer Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Marketable Securities Fair Value Total Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Marketable Securities Unrealized Losses Total Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss Number of securities Number of Available For Sale Securities In Continuous Unrealized Loss Position Number of available for sale securities in continuous unrealized loss position. Retirement Benefits, Description [Abstract] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Retirement Plans Pension Plan [Member] Other Postretirement Benefit Plans Other Postretirement Benefits Plan [Member] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan, Employer Contribution Defined Benefit Plan, Plan Assets, Contributions by Employer Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year Commitments and Contingencies Disclosure [Abstract] Site Contingency [Table] Site Contingency [Table] Loss Contingency Nature [Axis] Loss Contingency Nature [Axis] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] Four Corners Coal Plant Participants Four Corners Coal Plant Participants [Member] Four Corners Coal Plant Participants [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Loss Contingency, Civil Penalty Amount Loss Contingency, Civil Penalty Amount Loss Contingency, Civil Penalty Amount Loss Contingency Environmental Mitigation Project Cost Loss Contingency Environmental Mitigation Project Cost Loss Contingency For Environmental Mitigation Project Cost Accrual for Environmental Loss Contingencies Accrual for Environmental Loss Contingencies AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest Rate Contract Interest Rate Contract [Member] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income [Member] Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax Reclassification from AOCI, Current Period, Tax Reclassification from AOCI, Current Period, Tax Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Net realized gain on sale of securities Gain (Loss) on Investments, Excluding Other than Temporary Impairments Income (loss) before income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Interest on long-term debt and revolving credit facility Interest Income (Expense), Nonoperating, Net Income tax expense (benefit) Income Tax Expense (Benefit) Net income (loss) Net Income (Loss) Attributable to Parent Income Tax Disclosure [Abstract] Income Tax [Table] Income Tax [Table] Income Tax [Table] Tax Period [Axis] Tax Period [Axis] Tax Period [Domain] Tax Period [Domain] Income Tax [Line Items] Income Tax [Line Items] Income Tax [Line Items] Effective tax rate Effective Income Tax Rate Reconciliation, Percent Federal statutory tax rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Income Tax Rate Capital Gains Income Tax Rate Capital Gains Percentage of federal tax rate applicable to capital gains. Period in which adjustments related to the Federal Tax Cuts and Jobs Act of 2017 needed to be made Period in which adjustments related to the Federal Tax Cuts and Jobs Act of 2017 needed to be made Period in which adjustments related to the Federal Tax Cuts and Jobs Act of 2017 needed to be made, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Tax Year Closed Tax Year Closed Tax year no longer subject to tax examination by the taxing authorities. Revenue Recognition, Policy Revenue Recognition, Policy [Policy Text Block] Fair Values Of Long-Term Debt And Short-Term Borrowings Under the RCF Fair Value, by Balance Sheet Grouping [Table Text Block] Unrecognized Gross Unrealized Gains (Losses) And the Fair Value Available-for-sale Securities [Table Text Block] Unrealized Holding Losses Included In Pre-Tax Income Schedule of Unrealized Holding Losses Included In Pre Tax Income [Table Text Block] Schedule of Unrealized Holding Losses Included In Pre Tax Income [Table Text Block] Marketable Securities Fair Value and Unrecognized Gross Unrealized Gain Marketable Securities [Table Text Block] Contractual Year For Maturity Of Available-For-Sale Securities Schedule Of Contractual Year For Maturity Of Available For Sale Securities [Text Block] Schedule Of Contractual Year For Maturity Of Available For Sale Securities [Text Block] Sale of Securities And The Related Effects On Pre-Tax Income Schedule Of Sale Of Securities And The Related Effects On Pre Tax Income [Text Block] Schedule Of Sale Of Securities And The Related Effects On Pre Tax Income [Text Block] Unrealized Gain (Loss) on Investments Unrealized Gain (Loss) on Investments [Table Text Block] Fair Value, Measurement Inputs, Disclosure Fair Value Inputs, Assets, Quantitative Information [Table Text Block] Cash Cash [Member] International Equity Securities International Equity Securities [Member] International Equity Securities [Member] Decommisioning Fund Investments Decommisioning Fund Investments [Member] Decommisioning Fund Investments [Member] Held in trust Decommissioning Fund Investment Held in trust Decommissioning Fund Investment [Member] Held in trust Decommissioning Fund Investment Available-for-sale Securities, Unrealized Gain Position, Fair Value Available-for-sale Securities, Unrealized Gain Position, Fair Value This item represents the aggregate fair value of investments in debt and equity securities in an unrealized gain position which are categorized neither as held-to-maturity nor trading securities. Available-for-sale Securities, Gross Unrealized Gain Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax Marketable Securities Fair Value, Total Marketable Securities Fair Value, Total This item represents the aggregate fair value of investments in debt and equity securities in an unrealized gain position. This total also includes other current assets. Cash Cash Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Nuclear Fuel Nuclear Fuel [Member] ASSETS Assets [Abstract] Utility plant: Public Utilities, Property, Plant and Equipment, Net [Abstract] Electric plant in service Public Utilities, Property, Plant and Equipment, Plant in Service Less accumulated depreciation and amortization Public Utilities, Property, Plant and Equipment, Accumulated Depreciation Net plant in service Property, Plant and Equipment, Net Construction work in progress Public Utilities, Property, Plant and Equipment, Construction Work in Progress Nuclear fuel; includes fuel in process of $69,533 and $59,689, respectively Public Utilities, Property, Plant and Equipment, Fuel Net nuclear fuel Nuclear Fuel, Net of Amortization Net utility plant Public Utilities, Property, Plant and Equipment, Net Current assets: Assets, Current [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Accounts receivable, principally trade, net of allowance for doubtful accounts of $1,820 and $2,300, respectively Accounts Receivable, Net, Current Inventories, at cost Public Utilities, Inventory Prepayments and other Prepaid Expense, Current Total current assets Assets, Current Deferred charges and other assets: Regulated Entity, Other Assets, Noncurrent [Abstract] Decommissioning trust funds Decommissioning Fund Investments Regulatory assets Regulatory Assets, Noncurrent Other Other Assets, Noncurrent Total deferred charges and other assets Regulated Entity, Other Assets, Noncurrent Total assets Assets CAPITALIZATION AND LIABILITIES Liabilities and Equity [Abstract] Capitalization: Capitalization, Long-term Debt and Equity [Abstract] Common stock, stated value $1 per share, 100,000,000 shares authorized, 65,666,420 and 65,694,829 shares issued, and 162,268 and 133,859 restricted shares, respectively Common Stock, Value, Issued Capital in excess of stated value Additional Paid in Capital, Common Stock Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive income (loss), net of tax Total stockholders' equity subtotal before treasury stock Stockholders' Equity before Treasury Stock Treasury stock, 25,166,943 and 25,244,350 shares, respectively, at cost Treasury Stock, Value Common stock equity Stockholders' Equity Attributable to Parent Long-term debt, net of current portion Long-term Debt, Excluding Current Maturities Total capitalization Capitalization, Long-term Debt and Equity Current liabilities: Liabilities, Current [Abstract] Short-term borrowings under the revolving credit facility Line of Credit, Current Accounts payable, principally trade Accounts Payable, Trade, Current Taxes accrued Taxes Payable, Current Interest accrued Interest Payable, Current Over-collection of fuel revenues Deferred Fuel Costs Over Collected Carrying amount as of the balance sheet date of capitalized fuel costs of a regulated entity that are expected to be refunded through rate adjustments within one year or the normal operating cycle, if longer. Other Other Liabilities, Current Total current liabilities Liabilities, Current Deferred credits and other liabilities: Deferred Credits and Other Liabilities [Abstract] Accumulated deferred income taxes Deferred Tax Liabilities, Net, Noncurrent Accrued pension liability Liability, Defined Benefit Pension Plan, Noncurrent Accrued post-retirement benefit liability Liability, Other Postretirement Defined Benefit Plan, Noncurrent Asset retirement obligation Asset Retirement Obligations, Noncurrent Regulatory liabilities Regulatory Liability, Noncurrent Other Other Liabilities, Noncurrent Total deferred credits and other liabilities Liabilities, Other than Long-term Debt, Noncurrent Commitments and contingencies Commitments and Contingencies Total capitalization and liabilities Liabilities and Equity Dividends, Common Stock [Abstract] Dividends, Common Stock, Cash Dividends, Common Stock, Cash Public Utility, Property, Plant and Equipment [Table] Public Utility, Property, Plant and Equipment [Table] Public Utilities, Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Public Utilities, Regulatory Rate Making Impact Public Utilities, Regulatory Rate Making Impact [Member] Public Utilities, Regulatory Rate Making Impact PUCT Docket No. 46831 PUCT Docket No. 46831 [Member] PUCT Docket No. 46831 [Member] PUCT Docket No. 46610 PUCT Docket No. 46610 [Member] PUCT Docket No. 46610 PUCT Docket No. 46308 PUCT Docket No. 46308 [Member] PUCT Docket No. 46308 NMPRC Case No. 15-00127-UT NMPRC Case No. 15-00127-UT [Member] NMPRC Case No. 15-00127-UT NMPRC Case No. 18-00016-UT NMPRC Case No. 18-00016-UT [Member] NMPRC Case No. 18-00016-UT [Member] Regulatory Matters [Axis] Regulatory Matters [Axis] Regulatory Matters Regulatory Matters [Domain] Regulatory Matters [Domain] Regulatory Matters Non-Fuel Base Rate Non-Fuel Base Rate [Member] Non-Fuel Base Rate 2016 Energy Efficiency Programs 2016 Energy Efficiency Programs [Member] 2016 Energy Efficiency Programs Fixed Fuel Factor Fixed Fuel Factor [Member] Fixed Fuel Factor Utility Cost Of Service Utility Cost Of Service [Member] Utility cost of service 2017 Energy Efficiency Programs 2017 Energy Efficiency Programs [Member] 2017 Energy Efficiency Programs 2015 Energy Efficiency Programs 2015 Energy Efficiency Programs [Member] 2015 Energy Efficiency Programs Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] PUCT PUCT [Member] PUCT [Member] NMPRC NMPRC [Member] NMPRC [Member] FERC FERC [Member] FERC [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Four Corners decommissioning [Member] Four Corners decommissioning [Member] Four Corners decommissioning Member Four Corners Generating Station Four Corners Generating Station [Member] Four Corners Generating Station [Member] Public Utility [Axis] Public Utility [Axis] Utility Plant [Domain] Utility Plant [Domain] Community Solar At Montana Power Station Community Solar At Montana Power Station [Member] Community Solar At Montana Power Station [Member] Community Solar at Newman Community Solar at Newman [Member] Community Solar at Newman [Member] Solar Facility At Newman Power Station Solar Facility At Newman Power Station [Member] Solar Facility At Newman Power Station [Member] Holloman Holloman [Member] Holloman [Member] Public Utilities, Approved Rate Increase (Decrease), Amount Public Utilities, Approved Rate Increase (Decrease), Amount Regulatory Current Asset, End Date for Recovery Regulatory Current Asset, End Date for Recovery Recovery of rate case expenses Regulatory Assets Period To Relate Back New Rates Period To Relate Back New Rates Time period given to public utility (by statutory right) to relate back for consumption, its new rates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Public Utilities, Requested Rate Decrease, Amount Public Utilities, Requested Rate Increase (Decrease), Amount Surcharge Or Refund Period Surcharge Or Refund Period Time period given to public utility (by regulatory agency) to surcharge or to refund customers, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Minimum Distributive Generation Charge Minimum Distributive Generation Charge Minimum monthly bill charge for distributive generation customers. Nuclear Decommissioning Funding, Annual Recovery Payments to Acquire Investments to be Held in Decommissioning Trust Fund Requested Depreciation Expense, Decrease Public Utilities, Increase (Decrease) In Requested Depreciation Expense Amount by which public utilities requested depreciation expense is increased (decreased). Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program Public Utilities, Requested Performance Incentive Bonus Related To Energy Efficiency Program Increase (Decrease) In Fixed Fuel Factor, Percentage Public Utilities, Approved Rate Increase (Decrease), Percentage Public Utilities, Requested Rate Increase (Decrease), Percentage Public Utilities, Requested Rate Increase (Decrease), Percentage Reconcilable Fuel Expense Reconcilable Fuel Expense Reconcilable Fuel Expense Fuel Reconciliation Period Fuel Reconciliation Period Fuel reconciliation period Fuel reconciliation effect, net amount Public Utilities, Fuel Reconciliation Effect, Net Amount Amount of public utilities fuel reconciliation effect, net. Electric Capacity Electric Capacity Electric Capacity Number of customers subscribed Number Of Customers Represents the number of customers. Property, Plant and Equipment, Operational Date Property, Plant and Equipment, Operational Date Date the public utility's property, plant and equipment became operational, in CCYY-MM-DD format. Electric capacity, requested Electric capacity, requested Electric capacity, requested Public Utilities, Approved Return on Equity, Percentage Public Utilities, Approved Return on Equity, Percentage Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program Show Cause Orders issued, Number Show Cause Orders issued, Number Show cause order issued by the Federal Energy Regulatory Commission Number of public utilities affected Number of public utilities affected Number of public utilities affected Fuel Over And Under Materiality Threshold Percentage Fuel Over And Under Materiality Threshold Percentage Fuel Over And Under Materiality Threshold Percentage. Palo Verde Performance Award Period Palo Verde Performance Award Period Palo Verde Performance Award Period Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Period to comply with order from regulatory agency Period to comply with order from regulatory agency Period to comply with regulatory agency order Decommissioning Costs Increase (Decrease) Decommissioning Costs Increase (Decrease) Decommissioning costs increase (decrease) Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Retail Retail [Member] Retail [Member] Wholesale Wholesale [Member] Wholesale [Member] Transmission Transmission [Member] Transmission [Member] Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Asset [Domain] Regulatory Asset [Domain] Recovery period per ASC 980-605-25 Recovery period per ASC 980-605-25 [Member] Recovery period per ASC 980-605-25 [Member] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Customer Payment Terms Customer Payment Terms Amount of time, in days, that customers have to pay the Company for services provided Number Of Customers Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period US Treasury and Government Accounting Standards Update 2016-09 Accounting Standards Update 2016-09 [Member] Accounting Standards Update 2014-09 Accounting Standards Update 2014-09 [Member] Accounting Standards Update 2016-01 Accounting Standards Update 2016-01 [Member] Accounting Standards Update 2016-15 Accounting Standards Update 2016-15 [Member] Potential Impact of Adopting Accounting Standards Update 2018-02 Potential Impact of Adopting Accounting Standards Update 2018-02 [Member] Potential Impact of Adopting Accounting Standards Update 2018-02 [Member] Increase in operating loss carryforward deferred tax assets Deferred Tax Assets, Operating Loss Carryforwards Cost of reimbursable expense Cost of Reimbursable Expense Increase in retained earnings Unrealized loss on securities Unrealized Loss on Securities Proceeds from insurance settlement, investing activities Proceeds from Insurance Settlement, Investing Activities Decrease in accumulated other comprehensive income Accounting Policies [Abstract] Accrued unbilled revenues Unbilled Receivables, Current Document Information [Table] Document Information, Document [Axis] Document Information, Document [Axis] Document [Domain] Document [Domain] Document and Entity Information [Line Items] Document Information [Line Items] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Trading Symbol Trading Symbol Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Disaggregation of Revenue [Abstract] Revenues from contracts with customers Revenue from Contract with Customer, Excluding Assessed Tax Other Other Revenue, Net Total operating revenues Electric Revenue Revenue Recognition Disclosure Revenue from Contract with Customer [Text Block] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Internal Revenue Service (IRS) Internal Revenue Service (IRS) [Member] TEXAS TEXAS NEW MEXICO NEW MEXICO ARIZONA ARIZONA Latest Tax Year Latest Tax Year [Member] Revenues [Abstract] Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Company Share of Recovered Costs Company Share of Recovered Costs [Table Text Block] Company Share of Recovered Costs from DOE settlement. [Table Text Block] Schedule of Jointly Owned Utility Plants Schedule of Jointly Owned Utility Plants [Table Text Block] Disclosure Text Block [Abstract] Litigation Matters Disclosure Legal Matters and Contingencies [Text Block] Commitments, Contingencies And Uncertainties Commitments and Contingencies Disclosure [Text Block] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract] Net income (loss) Adjustments to reconcile net income (loss) to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization of electric plant in service Utilities Operating Expense, Depreciation and Amortization Amortization of Nuclear Fuel Amortization of Nuclear Fuel Lease Deferred income taxes, net Deferred Income Tax Expense (Benefit) Allowance for equity funds used during construction Increase (Decrease) in Allowance for Equity Funds Used During Construction Other amortization and accretion Amortization of Other Deferred Charges Net loss (gain) on decommissioning trust funds Marketable Securities, Gain (Loss), Excluding Other than Temporary Impairments Other operating activities Other Operating Activities, Cash Flow Statement Change in: Increase (Decrease) in Operating Capital [Abstract] Accounts receivable Increase (Decrease) in Accounts Receivable Inventories Increase (Decrease) in Inventories Prepayments and other Increase (Decrease) in Prepaid Expense Accounts payable Increase (Decrease) in Accounts Payable Taxes accrued Increase (Decrease) in Accrued Taxes Payable Interest accrued Increase (Decrease) in Interest Payable, Net Net over-collection of fuel revenues Other current liabilities Increase (Decrease) in Other Operating Liabilities Deferred charges and credits Increase (Decrease) in Deferred Charges Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities, Continuing Operations Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract] Cash additions to utility property, plant and equipment Payments to Acquire Property, Plant, and Equipment Cash additions to nuclear fuel Payments for (Proceeds from) Nuclear Fuel Insurance proceeds received for equipment Capitalized interest and AFUDC: Interest Costs Incurred [Abstract] Utility property, plant and equipment Allowance for Funds Used During Construction, Investing Activities Nuclear fuel and other Interest Costs Capitalized Allowance for equity funds used during construction Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity Decommissioning trust funds: Decommissioning Trust Fund Abstract Decommissioning Trust Fund [Abstract] Purchases, including funding of $0.5 million and $1.1 million, respectively Sales and maturities Proceeds from Decommissioning Trust Fund Assets Other investing activities Payments for (Proceeds from) Other Investing Activities Net cash used for investing activities Net Cash Provided by (Used in) Investing Activities, Continuing Operations Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities, Continuing Operations [Abstract] Dividends paid Payments of Ordinary Dividends, Common Stock Borrowings under the revolving credit facility: Proceeds from (Repayments of) Long-term Debt and Capital Securities [Abstract] Proceeds Proceeds from Unsecured Lines of Credit Payments Repayments of Lines of Credit Other financing activities Proceeds from (Payments for) Other Financing Activities Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities, Continuing Operations Net decrease in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] RGRT Senior Notes Debt Obligations by Purpose [Axis] Debt Obligations by Purpose [Axis] Debt Obligations by Purpose [Axis] Debt Obligation By Purpose [Domain] Debt Obligation By Purpose [Domain] Debt Obligation By Purpose [Domain] Nuclear Fuel Financing Nuclear Fuel Financing [Member] Nuclear Fuel Financing [Member] Working Capital and General Purpose Working Capital and General Purpose [Member] Working Capital and General Purpose [Member] Financial Instruments and Investments [Line Items] Long-term Debt, Gross Long-term Debt, Gross Line of Credit Facility, Amount Outstanding Long-term Line of Credit Decommissioning Fund Investments Class of Stock Disclosures [Abstract] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities Excluded from Computation of Earnings Per Share, by Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Performance Shares Antidilutive Securities Excluded from the Computation of Diluted Earnings Per Share, Amount Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Performance shares payout based upon performance Performance shares payout based upon performance Performance shares payout Performance shares payout level Performance Shares Payout Level Performance Shares Payout Level Statement of Comprehensive Income [Abstract] Other comprehensive income (loss): Other Comprehensive Income (Loss), before Tax [Abstract] Unrecognized pension and post-retirement benefit costs: Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax [Abstract] Net gain (loss) arising during period Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax Prior service benefit Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax Reclassification adjustments included in net income for amortization of: Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax [Abstract] Prior service benefit Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax Net loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Net unrealized gains/loss on marketable securities: Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax, [Abstract] Net holding gains (losses) arising during period Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax Reclassification adjustments for net (gains) losses included in net income Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax Net losses on cash flow hedges: Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax [Abstract] Reclassification adjustment for interest expense included in net income Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax Total other comprehensive income (loss) before income taxes Other Comprehensive Income (Loss), before Tax Income tax benefit (expense) related to items of other comprehensive income (loss): Other Comprehensive Income (Loss), Tax [Abstract] Unrecognized pension and post-retirement benefit costs Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Net unrealized (gains) losses on marketable securities Other Comprehensive Income (Loss), Available-for-sale Securities, Tax Losses on cash flow hedges Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax Total income tax benefit (expense) Other Comprehensive Income (Loss), Tax Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Attributable to Parent Palo Verde Operations and Maintenance Expense [Abstract] Palo Verde Operations and Maintenance Expense [Abstract] Jointly Owned Utility Plant Interests [Table] Jointly Owned Utility Plant Interests [Table] Utilities Operating Expense, Maintenance and Operations Comprehensive Income (Loss) Note Comprehensive Income (Loss) Note [Text Block] Principles Of Preparation Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Income Taxes Income Tax Disclosure [Text Block] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Antidilutive Securities Excluded From Computation Of Earnings Per Share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Service cost Interest cost Defined Benefit Plan, Interest Cost Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Amortization of net (gain) loss Defined Benefit Plan, Amortization of Gain (Loss) Amortization of prior service benefit Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Net periodic benefit cost (benefit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Schedule of Net Benefit Costs Schedule of Net Benefit Costs [Table Text Block] Financial Instruments and Investments Fair Value Disclosures [Text Block] Available-for-sale Securities Available-for-sale Securities [Member] Fair Value, Assets Measured on Recurring Basis, Disclosure Items [Axis] Fair Value, Hierarchy [Axis] Fair Value, Assets Measured on Recurring Basis, Disclosure Items [Domain] Fair Value Hierarchy [Domain] Quoted Prices In Active Markets For Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Trading Securities Trading Securities Decommissioning Fund Investments, Fair Value Decommissioning Fund Investments, Fair Value Available-for-sale Securities Available-for-sale Securities Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Property, Plant and Equipment, Useful Life Property, Plant and Equipment, Useful Life Employee Benefits Pension and Other Postretirement Benefits Disclosure [Text Block] Fair Value, Assets, Level 1 to Level 2 Transfers, Description Fair Value, Assets, Level 1 to Level 2 Transfers, Description Fair Value, Assets, Level 2 to Level 1 Transfers, Description Fair Value, Assets, Level 2 to Level 1 Transfers, Description Fair Value, Asset, Level 3 Purchases, (Sales), Issuances, (Settlements) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, (Sales), Issuances, (Settlements) Financial Instruments And Investments [Abstract] Financial Instruments and Investments Unrealized losses included in pre-tax income Unrealized Gains and losses recognized [Table] Unrealized Gains and losses recognized [Table] Unrealized Gains and losses recognized [Table] Unrealized Gains and losses recognized [Line Items] Unrealized Gains and losses recognized [Line Items] [Line Items] for Unrealized Gains and losses recognized [Table] Net gains and (losses) recognized on equity securities Net gains and (losses) recognized on equity securities Net gains and (losses) recognized on equity securities Less: Net (gains) and losses recognized on equity securities sold Net (gains) and losses recognized on equity securities sold Net gains and (losses) recognized on equity securities sold Unrealized gains and (losses) recognized on equity securities still held at reporting date Unrealized Gain (Loss) on Securities Reclassification, Policy Reclassification, Policy [Policy Text Block] Use of Estimates, Policy Use of Estimates, Policy [Policy Text Block] Depreciation, Depletion, and Amortization, Policy Depreciation, Depletion, and Amortization [Policy Text Block] New Accounting Standards, Policy New Accounting Pronouncements, Policy [Policy Text Block] Provision for Doubtful Accounts Provision for Doubtful Accounts Common Stock and Earnings Per Share Information Earnings Per Share [Text Block] Operating revenues Energy expenses: Energy Expenses Abstract Energy Expenses [Abstract] Fuel Utilities Operating Expense, Fuel Used Purchased and interchanged power Utilities Operating Expense, Purchased Power Cost of services, energy services Cost of Services, Energy Services Operating revenues net of energy expenses Revenues Net Energy Expenses Total revenues less energy expenses. Other operating expenses: Utilities Operating Expense [Abstract] Depreciation and amortization Taxes other than income taxes Utilities Operating Expense, Taxes Utilities operating expense Utilities Operating Expense Operating income Operating Income (Loss) Other income (deductions): Nonoperating Income (Expense) [Abstract] Allowance for equity funds used during construction Miscellaneous non-operating deductions Nonoperating income Nonoperating Income (Expense) Interest charges (credits): Interest Expense [Abstract] Interest on long-term debt and revolving credit facility Interest Expense, Other Long-term Debt Capitalized interest Interest Costs Capitalized Adjustment Allowance for borrowed funds used during construction Public Utilities, Allowance for Funds Used During Construction, Additions Interest expense Interest Expense Income (loss) before income taxes Income tax (benefit) expense Basic earnings (loss) per share Earnings Per Share, Basic Diluted earnings (loss) per share Earnings Per Share, Diluted Dividends declared per share of common stock Common Stock, Dividends, Per Share, Declared Weighted average number of shares outstanding Weighted Average Number of Shares Outstanding, Basic Weighted average number of shares and dilutive potential shares outstanding Weighted Average Number of Shares Outstanding, Diluted Earnings Per Share [Abstract] Basic number of common shares outstanding Dilutive effect of unvested performance awards Incremental Common Shares Attributable to Dilutive Effect of Nonvested Shares with Forfeitable Dividends Diluted number of common shares outstanding Income allocated to participating restricted stock Undistributed Earnings (Loss) Allocated to Participating Securities, Basic Net income (loss) available to common shareholders, basic Net Income (Loss) Available to Common Stockholders, Basic Income reallocated to participating restricted stock Undistributed Earnings (Loss) Allocated to Participating Securities, Diluted Net income (loss) available to common shareholders, diluted Net Income (Loss) Available to Common Stockholders, Diluted Earnings Per Share, Basic, Distributed Earnings Per Share, Basic, Distributed Earnings (losses) Per Share, Basic, Undistributed Earnings Per Share, Basic, Undistributed Basic net income (loss) per common share Earnings Per Share, Diluted, Distributed Earnings Per Share, Diluted, Distributed Earnings (losses) Per Share, Diluted, Undistributed Earnings Per Share, Diluted, Undistributed Diluted net income (loss) per common share