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Regulatory and Rate Matters - TNMP Narrative (Details) advanced_meter in Thousands, $ in Thousands |  |  |  |  |  |  |  | 1 Months Ended | 12 Months Ended |  |  |  |
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Feb. 05, 2021 USD ($) | Jun. 26, 2020 USD ($) | Apr. 06, 2020 USD ($) | Dec. 20, 2018 USD ($) | May 30, 2018 USD ($) | Jan. 10, 2018 USD ($) | Dec. 07, 2016 USD ($) | Oct. 31, 2019 USD ($) | Aug. 31, 2019 USD ($) | Jul. 31, 2011 USD ($) | Dec. 31, 2020 USD ($) advanced_meter | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Oct. 02, 2020 USD ($) | Jul. 27, 2020 USD ($) | May 29, 2020 USD ($) | Dec. 31, 2019 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net increase (decrease) in regulatory liabilities |  |  |  |  |  |  |  |  |  |  |  | $ 7,175 |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities |  |  |  |  |  |  |  |  |  |  | $ 670,292 |  |  |  |  |  | $ 683,769
Net increase (decrease) in regulatory liabilities |  |  |  |  |  |  |  |  |  |  |  | 11,244 |  |  |  |  |
Texas-New Mexico Power Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities |  |  |  |  |  |  |  |  |  |  | $ 187,407 |  |  |  |  |  | 182,979
Net increase (decrease) in regulatory liabilities |  |  |  |  |  |  |  |  |  |  |  | (4,069) |  |  |  |  |
Refund of regulatory liability to customers |  |  |  |  |  |  |  |  |  |  |  |  | $ 37,800 |  |  |  |
Estimate of possible upgrades |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 46,000 |  |  |
Energy efficiency cost recovery factor, requested change amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5,900 |
Energy efficiency cost recovery factor, requested performance bonus |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000 | $ 1,000 |
Requested increase annual distribution revenue requirement |  |  | $ 14,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incremental distribution investments |  |  | $ 149,200 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual distribution revenue requirement |  | $ 14,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
CTC Funding amount interest minimum |  |  |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |  |
Reduction to revenue |  |  |  |  |  |  |  |  |  |  |  | $ 5,400 |  |  |  |  |
Texas-New Mexico Power Company | 2018 TNMP Rate Case |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  | $ 10,000 | $ 25,900 |  |  |  |  |  |  |  |  |  |  |  |
Requested return on equity |  |  |  | 9.65% | 10.50% |  |  |  |  |  |  |  |  |  |  |  |
Requested cost of debt, percentage |  |  |  |  | 7.20% |  |  |  |  |  |  |  |  |  |  |  |
Requested debt capital structure, percentage |  |  |  | 55.00% | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Requested equity capital structure, percentage |  |  |  | 45.00% | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities |  |  |  |  |  |  |  |  |  |  |  |  | 146,500 |  |  |  |
Net increase (decrease) in regulatory liabilities |  |  |  |  |  |  |  |  |  |  |  |  | $ 14,400 |  |  |  |
Refund of federal income tax rates period |  |  |  |  |  |  |  |  |  |  |  | 3 years | 5 years |  |  |  |
Investments excluded from rate, amount |  |  |  | $ 10,600 |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery of direct costs |  |  |  |  |  |  |  | $ 3,300 | $ 3,800 |  |  |  |  |  |  |  |
Remaining recovery period of regulatory assets for which no return investment during recovery period is provided |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |
Write off regulatory dissallowance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500
Texas-New Mexico Power Company | Advanced Meter System Deployment and Surcharge Request |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved deployment costs |  |  |  |  |  |  |  |  |  | $ 113,400 |  |  |  |  |  |  |
Period of time to collect deployment costs through surcharge period |  |  |  |  |  |  |  |  |  | 12 years |  |  |  |  |  |  |
Number of advanced meters installed | advanced_meter |  |  |  |  |  |  |  |  |  |  | 242 |  |  |  |  |  |
Texas-New Mexico Power Company | Transmission Cost of Service Rates | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) | $ 112,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested increase in annual transmission service revenue | $ 14,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NMPRC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  |  | $ 99,200 |  |  |  |  |  |  |  |  |  |
Requested return on equity |  |  |  |  |  |  | 10.125% |  |  |  |  |  |  |  |  |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax |  |  |  |  |  |  |  |  |  |  |  |  | 23 years |  |  |  |
NMPRC | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  | $ 10,300 |  |  |  |  |  |  |  |  |  |  |
Requested return on equity |  |  |  |  |  | 9.575% |  |  |  |  |  |  |  |  |  |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |

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