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Fair Value of Derivative and Other Financial Instruments - Volume Positions and Requirements to Provide Collateral (Details) | 12 Months Ended
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Dec. 31, 2018 USD ($) MMBTU MWh | Dec. 31, 2017 USD ($) MMBTU MWh
Derivative [Line Items] |  |
Contract in a net liability position | $ | $ 0 | $ 0
Long | Commodity Contract | Economic Hedges | PNMR and PNM |  |
Derivative [Line Items] |  |
Power-related contracts | MMBTU | 100,000 | 100,000
Short | Commodity Contract | Economic Hedges | PNMR and PNM |  |
Derivative [Line Items] |  |
Power-related contracts | MWh | 0 | 0

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- Definition Contract With Customer, Liability, Net + References No definition available. + Details
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Namespace Prefix: | pnm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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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- Definition Nominal energy measure used to calculate payments on a derivative instrument. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:energyItemType
Balance Type: | na
Period Type: | duration

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

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- Details
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