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Equity Method Investment (Details) - NMRD $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2020 USD ($) MW | Mar. 31, 2019 USD ($)
Business Acquisition [Line Items] |  |
Renewable energy capacity in operating (in mw) | MW | 85.1 |
PNMR Development |  |
Business Acquisition [Line Items] |  |
Ownership percentage | 50.00% |
Contribution to construction activities | $ | $ 10.0 | $ 7.0

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- Definition Public Utilities, Contribution to Construction Activities + References No definition available. + Details
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Namespace Prefix: | pnm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Public Utilities, Ownership Percentage + References No definition available. + Details
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Namespace Prefix: | pnm_
Data Type: | num:percentItemType
Balance Type: | na
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- Definition Public Utilities, Renewable Energy Under Contract + References No definition available. + Details
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Namespace Prefix: | pnm_
Data Type: | num:powerItemType
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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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