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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 66,502 | $ 73,027 |
Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 16,714 | 19,940 |
Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 49,788 | 53,087 |
Public Service Company of New Mexico |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 60,859 | 64,401 |
Public Service Company of New Mexico | Coal |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | 16,714 | 19,940 |
Public Service Company of New Mexico | Materials and supplies |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory | $ 44,145 | $ 44,461 |
Public Service Company of New Mexico | Electric plant |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 2.52% | 2.33% | 2.27%
Public Service Company of New Mexico | Common, intangible, and general plant |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 8.36% | 5.40% | 4.66%
Texas-New Mexico Power Company |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used | 3.57% | 3.66% | 3.65%
Inventory | $ 5,643 | $ 8,626 |
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 | $ 5,643 | $ 8,626 |

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- Definition Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=68049868&loc=d3e3927-108312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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