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Commitments and Contingencies - Coal Supply (Details) - USD ($) $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2019 | Jun. 30, 2019 | Sep. 30, 2019 | Dec. 31, 2018 | Sep. 30, 2020 | Jan. 31, 2016
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Other current assets | $ 44,472 |  |  |  | $ 50,070 |
PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Other current assets | 36,561 |  |  |  | 37,661 |
PNM | Loss on long-term purchase commitment | Surface |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Final reclamation, capped amount to be collected |  |  |  | $ 100,000 |  |
Loss contingency accrual | 70,300 |  |  |  | 69,500 |
PNM | Loss on long-term purchase commitment | Underground |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Loss contingency accrual | 25,300 |  |  |  | 26,900 |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Estimate of possible loss |  |  |  |  | 89,500 |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Estimate of possible loss |  |  |  |  | 40,000 |
Coal supply | PNM | San Juan Generating Station |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Other current assets | 26,300 |  |  |  | 26,300 |
Coal supply | NM Capital | San Juan Generating Station |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Requirement to post reclamation bonds |  |  |  |  | 118,700 |
Cash used to support bank letter or credit arrangement |  |  |  |  | 30,300 | $ 30,300
Increase in coal mine decommissioning liability | PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Regulatory disallowance, additional amount recorded |  | $ 300 | $ 1,100 |  |  |
Increase in coal mine decommissioning liability | PNM | Underground and Surface |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Regulatory disallowance, additional amount recorded |  |  |  | 39,200 |  |
Increase in coal mine decommissioning liability | PNM | Underground |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Regulatory disallowance, additional amount recorded |  |  |  | $ 9,400 |  |
Mine Reclamation Trust | PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Required contribution to reclamation trust, current fiscal year |  |  |  |  | 8,100 |
Reclamation trust funding, year 2 |  |  |  |  | 10,900 |
Reclamation trust funding, year 3 |  |  |  |  | 11,700 |
Four Corners CSA | PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Other commitment, due in first year |  |  |  |  | 2,100 |
Other commitment, due in second year |  |  |  |  | 1,900 |
Other commitment, due in third year |  |  |  |  | 1,900 |
Other commitment, due in fourth year |  |  |  |  | 1,900 |
Other commitment, due in fifth year |  |  |  |  | $ 1,900 |
San Juan Generating Station | Loss on long-term purchase commitment | PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Annual funding post-term reclamation trust | $ 5,500 |  |  |  |  |

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