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Commitments and Contingencies - Coal Supply (Details) - USD ($) |  |  | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2016 | Jan. 31, 2016 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2018 | May 22, 2018 | Feb. 01, 2018 | Jan. 01, 2018 | Feb. 01, 2017
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Other current assets |  |  | $ 44,472,000 | $ 54,808,000 |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Other current assets |  |  | 36,561,000 | 43,516,000 |  |  |  |  |  |
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 | Loss on long-term purchase commitment | San Juan Generating Station |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Annual funding post-term reclamation trust |  |  | 5,500,000 | 10,000,000.0 | $ 5,800,000 |  |  |  |  |
Public Service Company of New Mexico | Mine Reclamation Trust | San Juan Generating Station |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Reclamation trust funding, next fiscal year |  |  | 9,700,000 |  |  |  |  |  |  |
Reclamation trust funding, year 2 |  |  | 10,900,000 |  |  |  |  |  |  |
Reclamation trust funding, year 3 |  |  | 11,700,000 |  |  |  |  |  |  |
Public Service Company of New Mexico | Mine Reclamation Trust | Four Corners |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Funds contributed to mine reclamation trust |  |  | 2,300,000 |  |  |  |  |  |  |
Funds contributed next fiscal year through year five |  |  | 1,900,000 |  |  |  |  |  |  |
Public Service Company of New Mexico | Surface | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Capped amount that can be collected from retail customers for final reclamation |  |  |  | 100,000,000.0 |  |  |  |  |  |
Liability for interim storage costs |  |  | 70,300,000 | 70,100,000 |  |  |  |  |  |
Public Service Company of New Mexico | Surface | Increase in coal mine decommissioning liability |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Net expense |  |  |  | 29,800,000 |  |  |  |  |  |
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 | Underground | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for interim storage costs |  |  | 25,300,000 | 23,200,000 |  |  |  |  |  |
Public Service Company of New Mexico | Underground | Increase in coal mine decommissioning liability |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Net expense |  |  |  | 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 |  |  | 92,600,000 |  |  |  |  |  |  |
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 |  |  | 40,000,000.0 |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Coal Supply |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | $ 26,300,000 |  |  |  |  |  |
NM Capital | BTMU Term Loan Agreement |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Long-term debt | $ 125,000,000.0 |  |  |  |  |  |  |  | $ 125,000,000.0 |
Repurchased face amount |  |  |  |  |  |  | $ 43,000,000.0 |  |  |
NM Capital | San Juan Generating Station | Coal Supply |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Funding provided | $ 125,000,000.0 | $ 125,000,000.0 |  |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  | 12.25% |  | 9.25%
Coal mine reclamation bonds to be posted with NMMMD |  |  | 118,700,000 |  |  |  |  |  |  |
Issuance in letters of credit |  | $ 30,300,000 | $ 30,300,000 |  |  |  |  |  |  |
NM Capital | San Juan Coal Company, Westmoreland | Coal Supply |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  | $ 50,100,000 |
Repurchased face amount |  |  |  |  |  |  | $ 50,100,000 |  |  |

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