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Fair Value of Derivative and Other Financial Instruments - Narrative (Details) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2020 USD ($) MW | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) MMBTU MW | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) MMBTU
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Obligations to return cash | $ 900,000 |  | $ 900,000 |  | $ 900,000
Contract in a liability position | 0 |  | 0 |  | 0
Debt securities, available-for-sale, unrealized loss position | $ 0 |  | 0 |  |
Other than temporary impairments |  |  | $ 0 |  |
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 right to reclaim cash | $ 0 |  | $ 0 |  | 0
Cash collateral under margin arrangements | 500,000 |  | 500,000 |  | $ 500,000
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings | 4,200,000 | $ 200,000 | $ (4,500,000) | $ 2,800,000 |
PNM | Commodity derivatives | Fair value hedging |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Economic Hedges (in mmbtu and mwh) | MMBTU |  |  | 0 |  | 0
PNM | Commodity derivatives | Designated as Hedging Instrument |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Other current assets | 1,097,000 |  | $ 1,097,000 |  | $ 1,089,000
PNM | Commodity derivatives | Designated as Hedging Instrument | Cost of energy |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Total gain | 100,000 | $ 100,000 | 100,000 | $ 100,000 |
PNM | Fuel and purchased power costs | Commodity derivatives | Designated as Hedging Instrument |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Other current assets | $ 0 |  | $ 0 |  | 0
PNM | Tri-State |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Power to be sold to third party (in megawatts) | MW | 100 |  | 100 |  |
Recurring | PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Available-for-sale debt securities | $ 408,551,000 |  | $ 408,551,000 |  | 388,832,000
Nuclear Decommissioning Trust | Recurring | PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Available-for-sale debt securities | 352,000,000.0 |  | 352,000,000.0 |  | 336,000,000.0
Mine Reclamation Trust | Recurring | PNM |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Available-for-sale debt securities | $ 56,600,000 |  | $ 56,600,000 |  | $ 52,800,000

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