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Income Taxes (Details) - USD ($) $ in Millions |  | 3 Months Ended
--- | --- | ---
May 23, 2017 | Mar. 31, 2019
Income Tax Contingency [Line Items] |  |
Effective tax rate |  | 9.42%
Tax benefits related to stock awards |  | $ 0.7
TNMP |  |
Income Tax Contingency [Line Items] |  |
Increase (decrease) in regulatory liability |  | $ 1.5
Effective tax rate |  | 8.41%
Tax benefits related to stock awards |  | $ 0.2
PNM |  |
Income Tax Contingency [Line Items] |  |
Effective tax rate |  | 11.89%
Tax benefits related to stock awards |  | $ 0.5
NMPRC |  |
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 | 21 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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