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Summary of the Business and Significant Accounting Policies - Inventories/Depreciation and Amortization (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 77,929 | $ 71,834 |
Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 24,914 | 22,777 |
Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 53,015 | 49,057 |
Public Service Company of New Mexico |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 72,225 | 67,097 |
Public Service Company of New Mexico | Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 24,914 | 22,777 |
Public Service Company of New Mexico | Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 47,311 | $ 44,320 |
Public Service Company of New Mexico | Electric plant |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 2.47% | 2.40% | 2.52%
Public Service Company of New Mexico | Common, intangible, and general plant |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 7.91% | 8.18% | 8.36%
Texas-New Mexico Power Company |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 4.04% | 3.49% | 3.57%
Inventory | $ 5,704 | $ 4,737 |
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 | $ 5,704 | $ 4,737 |

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