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Summary of the Business and Significant Accounting Policies - Narrative (Details) | 12 Months Ended
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Dec. 31, 2020 USD ($) utility | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Accounting Policies Disclosures [Line Items] |  |  |
Payment defaults | $ 0 |  |
Allowance for equity funds used during construction | $ 11,254,000 | $ 9,478,000 | $ 10,404,000
PNMR |  |  |
Accounting Policies Disclosures [Line Items] |  |  |
Number of regulated utilities | utility | 2 |  |
Restricted cash | $ 0 | 0 |
Public Service Company of New Mexico |  |  |
Accounting Policies Disclosures [Line Items] |  |  |
Restricted cash | 0 | 0 |
Allowance for funds used during construction, capitalized interest | $ 3,000,000.0 | $ 5,000,000.0 | $ 6,100,000
Allowance for funds used during construction, capitalized interest rate | 2.40% | 2.99% | 3.19%
Allowance for equity funds used during construction | $ 6,958,000 | $ 6,656,000 | $ 8,173,000
Allowance for funds used during construction, capitalized cost of equity rate | 3.42% | 3.95% | 4.25%
Impairment losses on securities held in the NDT | $ 3,200,000 | $ 5,700,000 | $ 13,700,000
Texas-New Mexico Power Company |  |  |
Accounting Policies Disclosures [Line Items] |  |  |
Restricted cash | 0 | 0 |
Allowance for funds used during construction, capitalized interest | $ 2,100,000 | $ 2,400,000 | $ 2,300,000
Allowance for funds used during construction, capitalized interest rate | 2.20% | 3.23% | 3.32%
Allowance for equity funds used during construction | $ 4,300,000 | $ 2,800,000 | $ 2,200,000
Allowance for funds used during construction, capitalized cost of equity rate | 4.42% | 3.78% | 3.29%

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- Definition Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 8A -URI http://asc.fasb.org/extlink&oid=121553693&loc=SL6284422-111562 + Details
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