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Income Taxes (Details) - USD ($) $ in Millions |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
May 23, 2017 | Mar. 31, 2018 | Mar. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2014 | Dec. 31, 2013
Income Tax Contingency [Line Items] |  |  |  |  |  |
New Mexico Corporate tax rate, 2014 |  |  |  |  |  | 7.60%
New Mexico Corporate tax rate, effective by 2018 |  |  |  |  | 5.90% |
Increase (decrease) in deferred tax assets as a result of tax rate changes |  |  | $ (0.1) |  |  |
Increase (decrease) in tax expense due to tax rate change (less than for 2017) |  |  | 0.1 |  |  |
Tax benefits related to stock awards |  | $ 1.3 |  |  |  |
Corporate and Other |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Increase (decrease) in tax expense due to tax rate change (less than for 2017) |  |  | (0.1) |  |  |
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 | 3 years |  |  |  |  |
TNMP |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Increase (decrease) in regulatory liability |  | 1.5 |  |  |  |
Tax benefits related to stock awards |  | 0.3 |  |  |  |
PNM |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Increase (decrease) in regulatory liability |  |  | $ (4.8) |  |  |
Tax benefits related to stock awards |  | $ 1.0 |  |  |  |
PNM | 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 | 3 years |  |  |  |  |
Forecast |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Effective tax rate |  |  |  | 12.11% |  |
Forecast | TNMP |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Effective tax rate |  |  |  | 23.47% |  |
Forecast | PNM |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Effective tax rate |  |  |  | 8.07% |  |

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