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Income Taxes (Details) - USD ($) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 10, 2018 | Sep. 30, 2020 | Sep. 30, 2020 | Dec. 31, 2017
Income Tax Contingency [Line Items] |  |  |  |
Accelerated refund application amount of AMT tax, CARES act |  | $ 5,200,000 | $ 5,200,000 |
Deferred federal, state and local, income tax expense (benefit) |  |  | $ 30,700,000 |
Effective tax rate |  |  | 8.44% |
Tax benefits related to stock awards |  | 0 | $ 400,000 |
PNM |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Deferred federal, state and local, income tax expense (benefit) |  |  | $ 21,600,000 |
Effective tax rate |  |  | 11.29% |
Tax benefits related to stock awards |  | 0 | $ 300,000 |
TNMP |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Deferred federal, state and local, income tax expense (benefit) |  |  | $ 9,100,000 |
Effective tax rate |  |  | 8.83% |
Tax benefits related to stock awards |  | $ 0 | $ 100,000 |
NMPRC |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Period of time for proposed return to customers the benefit of the reduction in federal corporate income tax rate |  |  |  | 23 years
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate |  |  |  | 3 years
NMPRC | PNM |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate | 3 years |  |  |

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