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Fair Value of Derivative and Other Financial Instruments - Defined Benefit Plans Disclosure (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Public Service Company of New Mexico | Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | $ 587,530 | $ 531,467 | $ 489,978
Public Service Company of New Mexico | Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 93,402 | 86,400 | 69,703
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: | Fair Value Measurement [Domain] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 498,907 | 445,984 |
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 587,891 | 532,659 |
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: | Estimate of Fair Value Measurement |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 88,984 | 86,675 |
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 93,710 | 86,749 |
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 1,310 | 1,022 |
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 92,400 | 85,727 |
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Plan | Participation in PNMR Master Trust Investments: |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 241,445 | 152,158 |
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 53,594 | 40,383 |
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 1,310 | 1,022 |
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 52,284 | 39,361 |
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Pension Plan | Participation in PNMR Master Trust Investments: |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 257,462 | 293,826 |
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 40,116 | 46,366 |
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 0 | 0 |
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 40,116 | 46,366 |
Texas-New Mexico Power Company | Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 66,149 | 59,367 | 55,074
Texas-New Mexico Power Company | Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 12,885 | 10,844 | $ 8,744
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: | Fair Value Measurement [Domain] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 56,966 | 49,353 |
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 66,196 | 59,327 |
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: | Estimate of Fair Value Measurement |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 9,230 | 9,974 |
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 12,861 | 10,910 |
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 18 | 275 |
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 12,843 | 10,635 |
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Plan | Participation in PNMR Master Trust Investments: |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 28,732 | 17,335 |
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 10,824 | 4,350 |
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 18 | 275 |
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 10,806 | 4,075 |
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Pension Plan | Participation in PNMR Master Trust Investments: |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 28,234 | 32,018 |
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 2,037 | 6,560 |
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 0 | 0 |
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 2,037 | 6,560 |
PNMR |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | $ 654,087 | $ 591,986 |
PNMR | Private equity funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] | us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember | us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember |
PNMR | Hedge funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] | us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember | us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember |
PNMR | Real estate funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List] | us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember | us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | $ 555,873 | $ 495,337 |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 20,812 | 19,982 |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | International funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 114,983 | 68,497 |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Domestic value |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 1,187 | 825 |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Domestic growth |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 173,931 | 172,326 |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | U.S. government |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 97,460 | 90,970 |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | International government |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 6,202 | 5,411 |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Municipals |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 6,277 | 6,980 |
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Corporate and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 135,021 | 130,346 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 270,177 | 169,493 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 20,812 | 19,982 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | International funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 114,983 | 68,497 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic value |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 135 | 0 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic growth |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 47,418 | 0 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. government |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 86,829 | 81,014 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | International government |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 0 | 0 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipals |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 0 | 0 |
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 0 | 0 |
PNMR | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 285,696 | 325,844 |
PNMR | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 0 | 0 |
PNMR | Significant Other Observable Inputs (Level 2) | International funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 0 | 0 |
PNMR | Significant Other Observable Inputs (Level 2) | Domestic value |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 1,052 | 825 |
PNMR | Significant Other Observable Inputs (Level 2) | Domestic growth |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 126,513 | 172,326 |
PNMR | Significant Other Observable Inputs (Level 2) | U.S. government |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 10,631 | 9,956 |
PNMR | Significant Other Observable Inputs (Level 2) | International government |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 6,202 | 5,411 |
PNMR | Significant Other Observable Inputs (Level 2) | Municipals |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 6,277 | 6,980 |
PNMR | Significant Other Observable Inputs (Level 2) | Corporate and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 135,021 | 130,346 |
PNMR | Fair Value Measured at Net Asset Value Per Share | Private equity funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 12,552 | 15,827 |
PNMR | Fair Value Measured at Net Asset Value Per Share | Hedge funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | 52,285 | 47,618 |
PNMR | Fair Value Measured at Net Asset Value Per Share | Real estate funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, plan assets, amount | $ 33,377 | $ 33,204 |

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