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Equity Method Investment (Details) - NMRD $ in Millions | 9 Months Ended
--- | ---
Sep. 30, 2020 USD ($) MW | Sep. 30, 2019 USD ($)
Business Acquisition [Line Items] |  |
Renewable energy capacity in operating (in mw) | MW | 135.1 |
PNMR Development |  |
Business Acquisition [Line Items] |  |
Ownership percentage | 50.00% |
Contribution to construction activities | $ | $ 23.3 | $ 29.3

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- Definition Public Utilities, Contribution to Construction Activities + References No definition available. + Details
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- Definition Public Utilities, Ownership Percentage + References No definition available. + Details
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- Definition Public Utilities, Renewable Energy Under Contract + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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Namespace Prefix: |
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Period Type: |

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- Details
Name: | dei_LegalEntityAxis=pnm_PNMRDevelopmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |