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Summary of the Business and Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2018
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
| 2018 |  | 2017 |  | 2016
PNM |  |  |  |  |
Electric plant | 2.40 | % |  | 2.52 | % |  | 2.33 | %
Common, intangible, and general plant | 8.18 | % |  | 8.36 | % |  | 5.40 | %
TNMP | 3.49 | % |  | 3.57 | % |  | 3.66 | %
Schedule of Inventory | Inventories consisted of the following at December 31:
| PNMR |  | PNM |  | TNMP
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
| (In thousands)
Coal | $ | 22,777 |  |  | $ | 16,714 |  |  | $ | 22,777 |  |  | $ | 16,714 |  |  | $ | — |  |  | $ | — |
Materials and supplies | 49,057 |  |  | 49,788 |  |  | 44,320 |  |  | 44,145 |  |  | 4,737 |  |  | 5,643 |
| $ | 71,834 |  |  | $ | 66,502 |  |  | $ | 67,097 |  |  | $ | 60,859 |  |  | $ | 4,737 |  |  | $ | 5,643 |

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