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Commitments and Contingencies - Coal Supply (Details) - USD ($) | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jan. 31, 2016
Other Commitments [Line Items] |  |  |  |  |  |
Other current assets | $ 64,549,000 | $ 64,549,000 | $ 44,472,000 |  |  |
Four Corners | Coal Supply |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Payments for relief from obligations |  | 75,000,000 |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Other current assets | 51,908,000 | 51,908,000 | 36,561,000 |  |  |
Public Service Company of New Mexico | Loss on long-term purchase commitment | San Juan Generating Station |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Annual funding post-term reclamation trust |  | 3,200,000 | 5,500,000 | $ 10,000,000.0 |  |
Public Service Company of New Mexico | Increase in coal mine decommissioning liability |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Net expense |  |  | 800,000 |  |  |
Public Service Company of New Mexico | Increase in coal mine decommissioning liability | Loss on long-term purchase commitment |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Estimated underpaid surface mining royalties under proposed rate change |  |  |  |  | $ 2,500,000 |
Public Service Company of New Mexico | Mine Reclamation Trust | San Juan Generating Station |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Reclamation trust funding, next fiscal year | 5,800,000 | 5,800,000 |  |  |  |
Reclamation trust funding, year 2 | 6,200,000 | 6,200,000 |  |  |  |
Reclamation trust funding, year 3 | 0 | 0 |  |  |  |
Public Service Company of New Mexico | Mine Reclamation Trust | Four Corners |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Funds contributed to mine reclamation trust |  | 2,000,000.0 | 2,300,000 | 2,300,000 |  |
Funds contributed next fiscal year through year five | 2,100,000 | 2,100,000 |  |  |  |
Public Service Company of New Mexico | Underground and Surface | Increase in coal mine decommissioning liability |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Net expense |  |  |  | 39,200,000 |  |
Public Service Company of New Mexico | Surface | Loss on long-term purchase commitment |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Liability for interim storage costs | 71,700,000 | 71,700,000 | 70,300,000 |  |  |
Public Service Company of New Mexico | Surface | Increase in coal mine decommissioning liability |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Net expense | 3,600,000 |  |  | 29,800,000 |  |
Public Service Company of New Mexico | Underground | Loss on long-term purchase commitment |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Liability for interim storage costs | 26,100,000 | 26,100,000 | 25,300,000 |  |  |
Public Service Company of New Mexico | Underground | Increase in coal mine decommissioning liability |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Net expense | 100,000 |  | 9,300,000 | $ 9,400,000 |  |
Public Service Company of New Mexico | San Juan Generating Station | Surface | Loss on long-term purchase commitment |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Estimate of possible loss | 80,100,000 | 80,100,000 |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Underground | Loss on long-term purchase commitment |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Estimate of possible loss | 35,100,000 | 35,100,000 |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Coal Supply |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Other current assets | 26,300,000 | 26,300,000 | $ 26,300,000 |  |  |
Public Service Company of New Mexico | Four Corners | Increase in coal mine decommissioning liability |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Net expense |  | 2,500,000 |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station And Four Corners | Surface |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Capacity to be collected for final reclamation |  | 100,000,000 |  |  |  |
NM Capital | San Juan Generating Station | Coal Supply |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Coal mine reclamation bonds to be posted with NMMMD | 118,700,000 | 118,700,000 |  |  |  |
Issuance in letters of credit | $ 30,300,000 | $ 30,300,000 |  |  |  | $ 30,300,000

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