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Summary of the Business and Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2017
Accounting Policies [Abstract] |
Schedule of Average Rates Used Allocated Between Depreciation Expense and Construction Expense Projects Based on Use of Equipment | Average straight-line rates used were as follows:
| Year ended December 31
| 2017 |  | 2016 |  | 2015
PNM |  |  |  |  |
Electric plant | 2.52 | % |  | 2.33 | % |  | 2.27 | %
Common, intangible, and general plant | 8.36 | % |  | 5.40 | % |  | 4.66 | %
TNMP | 3.57 | % |  | 3.66 | % |  | 3.65 | %
Schedule of Inventory | Inventories consisted of the following at December 31:
| PNMR |  | PNM |  | TNMP
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
| (In thousands)
Coal | $ | 16,714 |  |  | $ | 19,940 |  |  | $ | 16,714 |  |  | $ | 19,940 |  |  | $ | — |  |  | $ | — |
Materials and supplies | 49,788 |  |  | 53,087 |  |  | 44,145 |  |  | 44,461 |  |  | 5,643 |  |  | 8,626 |
| $ | 66,502 |  |  | $ | 73,027 |  |  | $ | 60,859 |  |  | $ | 64,401 |  |  | $ | 5,643 |  |  | $ | 8,626 |

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- Definition Tabular disclosure of the carrying amount as of the balance sheet date of merchandise, goods, commodities, or supplies held for future sale or to be used in manufacturing, servicing or production process. + 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)(b)) -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 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6361739&loc=d3e7789-107766 Reference 4: 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)(c)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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