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Income Taxes - Federal Income Tax Reform (Details) - USD ($) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Jan. 10, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Income Taxes [Line Items] |  |  |  |  |
Net increase (decrease) in regulatory liabilities |  |  |  | $ 7,175 |
Net decrease in deferred income tax liabilities (deferred income tax assets) |  |  |  | 1,910 |
Net increase in affiliate receivables (affiliate payables) |  |  |  | 0 |
Net deferred income tax expense |  |  |  | 5,265 |
Income taxes (benefit) |  | $ 20,636 | $ (25,282) | 7,775 |
Public Service Company of New Mexico |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net increase (decrease) in regulatory liabilities |  |  |  | 11,244 |
Net decrease in deferred income tax liabilities (deferred income tax assets) |  |  |  | (2,175) |
Net increase in affiliate receivables (affiliate payables) |  |  |  | 12,300 |
Net deferred income tax expense |  |  |  | 1,119 |
Income taxes (benefit) |  | 21,857 | (25,962) | (5,971) |
TNMP |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net increase (decrease) in regulatory liabilities |  |  |  | (4,069) |
Net decrease in deferred income tax liabilities (deferred income tax assets) |  |  |  | (9,784) |
Net increase in affiliate receivables (affiliate payables) |  |  |  | 4,042 |
Net deferred income tax expense |  |  |  | 1,673 |
Income taxes (benefit) |  | 6,308 | 5,046 | 16,880 |
Corporate and Other |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net increase (decrease) in regulatory liabilities |  |  |  | 0 |
Net decrease in deferred income tax liabilities (deferred income tax assets) |  |  |  | 13,869 |
Net increase in affiliate receivables (affiliate payables) |  |  |  | (16,342) |
Net deferred income tax expense |  |  |  | 2,473 |
Income taxes (benefit) |  | (7,529) | (4,366) | $ (3,134) |
NMPRC |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax |  |  |  |  | 23 years
NMPRC | Public Service Company of New Mexico |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax | 3 years |  |  |  |
New Mexico 2015 Rate Case | Public Service Company of New Mexico |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Pre-tax impairment of investments |  | 150,600 | 150,600 |  |
Income taxes (benefit) |  | $ 45,700 | $ (45,700) |  |

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