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Income Taxes (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | 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 regulatory liability | $ (4.8) | $ (7.1) |  |
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) |  |
Public Service Company of New Mexico |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Increase (decrease) in deferred tax assets as a result of tax rate changes | (0.1) | (0.7) |  |
Increase (decrease) in tax expense due to tax rate change (less than for 2017) | $ 0.1 | $ 0.8 |  |

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