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Summary of the Business and Significant Accounting Policies - Inventories/Depreciation and Amortization (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 66,417 | $ 77,929 |
Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 12,012 | 24,914 |
Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 54,405 | 53,015 |
Public Service Company of New Mexico |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 60,472 | 72,225 |
Public Service Company of New Mexico | Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 12,012 | 24,914 |
Public Service Company of New Mexico | Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 48,460 | $ 47,311 |
Public Service Company of New Mexico | Electric plant |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 2.47% | 2.47% | 2.40%
Public Service Company of New Mexico | Common, intangible, and general plant |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 7.65% | 7.91% | 8.18%
Texas-New Mexico Power Company |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 3.95% | 4.04% | 3.49%
Inventory | $ 5,945 | $ 5,704 |
Texas-New Mexico Power Company | Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 0 | 0 |
Texas-New Mexico Power Company | Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 5,945 | $ 5,704 |

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