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Income Taxes (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2017 | Dec. 31, 2020
Income Tax Contingency [Line Items] |  |  |  |  |
Accelerated refund application amount of AMT tax, CARES act |  |  |  |  | $ 5.2
Deferred tax assets, tax deferred expense, payroll taxes, CARES Act |  |  |  |  | $ 7.0
Refund amount of AMT tax, CARES act | $ 5.2 |  |  |  |
Deferred federal, state and local, income tax expense (benefit) |  |  | $ 24.5 |  |
Effective tax rate |  |  | 13.35% |  |
Tax benefits related to stock awards |  | $ 0.1 | $ 0.8 |  |
Tax benefits on merger-related costs |  | 0.3 | 1.6 |  |
PNM |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Deferred federal, state and local, income tax expense (benefit) |  |  | $ 15.2 |  |
Effective tax rate |  |  | 15.81% |  |
Tax benefits related to stock awards |  | 0.1 | $ 0.6 |  |
Tax benefits on merger-related costs |  | 0.1 | 0.1 |  |
TNMP |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Deferred federal, state and local, income tax expense (benefit) |  |  | $ 9.3 |  |
Effective tax rate |  |  | 10.64% |  |
Tax benefits related to stock awards |  | 0.1 | $ 0.2 |  |
Tax benefits on merger-related costs |  | $ 0.1 | $ 0.1 |  |
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 |

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