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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 71,834 | $ 66,502 |
Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 22,777 | 16,714 |
Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 49,057 | 49,788 |
Public Service Company of New Mexico |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 67,097 | 60,859 |
Public Service Company of New Mexico | Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 22,777 | 16,714 |
Public Service Company of New Mexico | Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 44,320 | $ 44,145 |
Public Service Company of New Mexico | Electric plant |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 2.40% | 2.52% | 2.33%
Public Service Company of New Mexico | Common, intangible, and general plant |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 8.18% | 8.36% | 5.40%
Texas-New Mexico Power Company |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 3.49% | 3.57% | 3.66%
Inventory | $ 4,737 | $ 5,643 |
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 | $ 4,737 | $ 5,643 |

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