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Income Taxes (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2016 | Mar. 31, 2016 | Mar. 31, 2015 | Sep. 30, 2016 | Dec. 31, 2015 | Dec. 31, 2013 | Dec. 31, 2012
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 |  | $ (7,100,000) | $ (2,000,000) |  |  |  |
Impairment of state net operating loss carryforward |  |  | 1,000,000 | $ 0 |  |  |
Interest income and interest expense applicable to federal income tax matters [Member] |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Net refunds from the IRS | $ 6,500,000 |  |  |  |  |  |
Increase (decrease) of interest receivable | (2,100,000) |  |  |  |  |  |
Interest income | 5,100,000 |  |  |  |  |  |
Interest expense | 700,000 |  |  |  |  |  |
Consulting expense fees | 900,000 |  |  |  |  |  |
Net settlement with taxing authority, pre-tax | 3,500,000 |  |  |  |  |  |
Corporate and Other [Member] |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Increase (decrease) in tax expense due to tax rate change |  | (100,000) | (200,000) |  |  |  |
Impairment of state net operating loss carryforward |  |  | 300,000 |  |  |  |
Corporate and Other [Member] | Interest income and interest expense applicable to federal income tax matters [Member] |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Net settlement with taxing authority, pre-tax, segment allocation | 600,000 |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Interest income and interest expense applicable to federal income tax matters [Member] |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Net settlement with taxing authority, pre-tax, segment allocation | 2,600,000 |  |  |  |  |  |
Texas-New Mexico Power Company [Member] | Interest income and interest expense applicable to federal income tax matters [Member] |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Net settlement with taxing authority, pre-tax, segment allocation | 300,000 |  |  |  |  |  |
Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Increase (decrease) in deferred tax assets as a result of tax rate changes |  | (700,000) | 700,000 |  |  |  |
Increase (decrease) in tax expense due to tax rate change |  | $ 800,000 | (500,000) |  |  |  |
Impairment of state net operating loss carryforward |  |  | $ 700,000 |  |  |  |
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Approved recovery of net operating loss carryforward | $ 2,100,000 |  |  |  |  |  |
Texas-New Mexico Power Company [Member] |  |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |  |
Impairment of state net operating loss carryforward |  |  |  |  | $ 0 |  |

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