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Income Taxes - Reconciliation of Unrecognized Tax Benefits (Expense) (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits, interest on income taxes expense | $ 0 | $ 0 | $ 0
PNMR |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | 10,693,000 | 10,194,000 | 9,429,000
Additions based on tax positions | 2,286,000 | 329,000 | 543,000
Additions (reductions) for tax positions of prior years | 173,000 | 170,000 | 222,000
Settlement payments | 0 | 0 | 0
Ending balance | 13,152,000 | 10,693,000 | 10,194,000
Unrecognized tax benefits that would impact effective tax rate | 10,700,000 |  |
Public Service Company of New Mexico |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | 7,776,000 | 7,288,000 | 6,563,000
Additions based on tax positions | 2,286,000 | 329,000 | 543,000
Additions (reductions) for tax positions of prior years | 168,000 | 159,000 | 182,000
Settlement payments | 0 | 0 | 0
Ending balance | 10,230,000 | 7,776,000 | 7,288,000
Unrecognized tax benefits that would impact effective tax rate | 7,800,000 |  |
Unrecognized tax benefits, interest on income taxes expense | 0 | 0 | 0
Texas-New Mexico Power Company |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | 114,000 | 103,000 | 63,000
Additions based on tax positions | 0 | 0 | 0
Additions (reductions) for tax positions of prior years | 5,000 | 11,000 | 40,000
Settlement payments | 0 | 0 | 0
Ending balance | 119,000 | 114,000 | 103,000
Unrecognized tax benefits that would impact effective tax rate | 100,000 |  |
Unrecognized tax benefits, interest on income taxes expense | $ 0 | $ 0 | $ 0

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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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 10B -URI http://asc.fasb.org/extlink&oid=120406818&loc=SL37586934-109318 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -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 settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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/disclosureRef -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 Amount of interest expense for an underpayment of income taxes. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319 + 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/disclosureRef -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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