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Income Taxes - Reconciliation of Unrecognized Tax Benefits (Expense) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
PNMR |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | $ 6,752 | $ 6,455 | $ 15,031
Additions based on tax positions | 262 | 242 | 1,214
Additions (reductions) for tax positions of prior years |  |  | (9,790)
Additions (reductions) for tax positions of prior years | 2,415 | 55 |
Settlement payments | 0 | 0 | 0
Ending balance | 9,429 | 6,752 | 6,455
Unrecognized tax benefits that would impact effective tax rate | 8,900 |  |
Public Service Company of New Mexico |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | 3,949 | 3,652 | 12,228
Additions based on tax positions | 262 | 242 | 1,214
Additions (reductions) for tax positions of prior years |  |  | (9,790)
Additions (reductions) for tax positions of prior years | 2,352 | 55 |
Settlement payments | 0 | 0 | 0
Ending balance | 6,563 | 3,949 | 3,652
Unrecognized tax benefits that would impact effective tax rate | 6,100 |  |
Texas-New Mexico Power Company |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | 0 | 0 | 0
Additions based on tax positions | 0 | 0 | 0
Additions (reductions) for tax positions of prior years |  |  | 0
Additions (reductions) for tax positions of prior years | 63 | 0 |
Settlement payments | 0 | 0 | 0
Ending balance | 63 | $ 0 | $ 0
Unrecognized tax benefits that would impact effective tax rate | $ 100 |  |

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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