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Income Taxes (Details) - USD ($) $ in Millions |  | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
May 23, 2017 | Mar. 31, 2017 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Income Tax Contingency [Line Items] |  |  |  |  |  |  |  |
New Mexico Corporate tax rate before change in 2013 |  |  |  |  |  |  |  | 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 |  |  |  |  | $ (0.7) |  |  |
Increase (decrease) in tax expense due to tax rate change (less than for 2017) |  |  |  |  | 0.8 |  |  |
Operating loss carryforward |  |  |  |  |  | $ 2.1 |  |
Corporate and Other |  |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |  |
Increase (decrease) in tax expense due to tax rate change (less than for 2017) |  | $ 0.1 |  |  | (0.1) |  |  |
Internal Revenue Service (IRS) |  |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |  |
Net refunds received from the IRS |  |  |  | $ 6.5 |  |  |  |
Increase (decrease) in interest receivable |  |  | $ (2.1) |  |  |  |  |
Interest income |  |  | 5.1 |  |  |  |  |
Interest expense |  |  | 0.7 |  |  |  |  |
Professional fees |  |  | $ 0.9 |  |  |  |  |
Net pre-tax impacts, liability (refund) |  |  |  | (3.5) |  |  |  |
Internal Revenue Service (IRS) | Corporate and Other |  |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |  |
Net pre-tax impacts, liability (refund) |  |  |  | (0.6) |  |  |  |
Internal Revenue Service (IRS) | PNM |  |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |  |
Net pre-tax impacts, liability (refund) |  |  |  | (2.6) |  |  |  |
Internal Revenue Service (IRS) | Texas-New Mexico Power Company |  |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |  |
Net pre-tax impacts, liability (refund) |  |  |  | $ 0.3 |  |  |  |
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 |  |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |  |
Increase (decrease) in regulatory liability |  | (4.8) |  |  | $ (7.1) |  |  |
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 |  |  |  |  |  |
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 |  |  |  |  |  |  |

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