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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. 06, 2020 USD ($) | Nov. 02, 2018 USD ($) | May 30, 2018 USD ($) | Jan. 10, 2018 USD ($) | Mar. 13, 2017 USD ($) | Dec. 07, 2016 USD ($) | Oct. 31, 2019 USD ($) | Aug. 31, 2019 USD ($) | Jul. 31, 2011 USD ($) | Dec. 31, 2019 USD ($) advanced_meter | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unrecovered investment revenue |  |  | $ 20,200 |  |  |  |  |  |  |  |  |
Net increase in regulatory liabilities |  |  |  |  |  |  |  |  |  |  | $ 7,175 | $ 548,952
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities |  |  |  |  |  |  |  |  |  | $ 683,769 | 719,946 |
Net increase in regulatory liabilities |  |  |  |  |  |  |  |  |  |  | 11,244 | 402,501
Texas-New Mexico Power Company |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities |  |  |  |  |  |  |  |  |  | $ 182,979 | 180,929 |
Net increase in regulatory liabilities |  |  |  |  |  |  |  |  |  |  | (4,069) | 146,451
Refund of regulatory liability to customers |  |  |  |  |  |  |  |  |  |  |  | 37,800
CTC Funding amount interest minimum |  |  |  |  |  |  |  |  |  | 15.00% |  |
Adjustment for collection of amortization |  |  |  |  | $ 1,100 |  |  |  |  |  |  |
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 |  | 6.44% | 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 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 |  |  |  |
Pre-tax write-off |  |  |  |  |  |  |  |  |  | $ 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) | $ 59,200 |  |  |  |  |  |  |  |  |  |  |
Requested increase in annual transmission service revenue | $ 7,800 |  |  |  |  |  |  |  |  |  |  |
NMPRC |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  | $ 99,200 |  |  |  |  |  |
Requested return on equity |  |  |  |  |  | 10.125% |  |  |  |  |  |
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% |  |  |  |  |  |  |  |

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