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Fair Value of Derivative and Other Financial Instruments - Narrative (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 USD ($) MW | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 MW
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Contract in a net liability position | $ 0 | $ 0 |  |
Available for sale securities realized impairment losses | $ (3,200,000) | 3,000,000.0 | $ (9,400,000) |
Minimum |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Investment allocation targets distribution period | 10 years |  |  |
Maximum |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Investment allocation targets distribution period | 15 years |  |  |
Equity Securities | Pension Plan |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Investment allocation targets | 35.00% |  |  |
Alternative Investments | Pension Plan |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Investment allocation targets | 15.00% |  |  |
Fixed income | Pension Plan |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Investment allocation targets | 50.00% |  |  |
PNM |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Expected exposure to market risk (in megawatts) | MW | 65 |  |  | 65
Power to be sold to third party (in megawatts) | MW | 36 |  |  | 36
Amounts recognized for the legal right to reclaim cash collateral | $ 0 | 0 |  |
Amounts posted as cash collateral under margin arrangements | 500,000 | 500,000 |  |
Obligations to return cash collateral | 900,000 | 900,000 |  |
PNM | Commodity Contract | Designated as hedging instrument |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Other current assets | 1,096,000 | 1,089,000 |  |
PNM | Commodity Contract | Designated as hedging instrument | Cost of Sales |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Gain (loss) on derivative | 100,000 | 100,000 |  |
PNM | Fuel and purchased power costs | Commodity Contract | Designated as hedging instrument |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Other current assets | $ 0 | 0 |  |
PNM | Tri-State |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Power to be sold to third party (in megawatts) | MW | 100 |  |  |
Measured on a recurring basis | PNM |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale | $ 440,115,000 | 388,832,000 |  |
Nuclear Decommissioning Trust | Measured on a recurring basis | PNM |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale | 379,200,000 | 336,000,000.0 |  |
Mine Reclamation Trust | Measured on a recurring basis | PNM |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale | $ 60,900,000 | $ 52,800,000 |  |

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