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Summary of the Business and Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2020
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,
| 2020 |  | 2019 |  | 2018
PNM |  |  |  |  |
Electric plant | 2.47 | % |  | 2.47 | % |  | 2.40 | %
Common, intangible, and general plant | 7.65 | % |  | 7.91 | % |  | 8.18 | %
TNMP | 3.95 | % |  | 4.04 | % |  | 3.49 | %
Schedule of Inventory | Inventories consisted of the following at December 31:
| PNMR |  | PNM |  | TNMP
| 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
| (In thousands)
Coal | $ | 12,012 |  |  | $ | 24,914 |  |  | $ | 12,012 |  |  | $ | 24,914 |  |  | $ | — |  |  | $ | — |
Materials and supplies | 54,405 |  |  | 53,015 |  |  | 48,460 |  |  | 47,311 |  |  | 5,945 |  |  | 5,704 |
| $ | 66,417 |  |  | $ | 77,929 |  |  | $ | 60,472 |  |  | $ | 72,225 |  |  | $ | 5,945 |  |  | $ | 5,704 |

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