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Commitments and Contingencies - Coal Supply (Details) - USD ($) | 1 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2020 | Jun. 30, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Jun. 30, 2020 | Jan. 31, 2016
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Other current assets | $ 64,549,000 | $ 60,138,000 | $ 64,549,000 |  |  |
PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Other current assets | 51,908,000 | 53,261,000 | 51,908,000 |  |  |
PNM | Loss on long-term purchase commitment | Surface |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Loss contingency accrual | 71,700,000 | 69,300,000 | 71,700,000 |  |  |
PNM | Loss on long-term purchase commitment | Underground |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Loss contingency accrual | 26,100,000 | 27,100,000 | 26,100,000 |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Estimate of possible loss |  | 76,800,000 |  |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Estimate of possible loss |  | 35,100,000 |  |  |  |
Coal supply | Four Corners |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Payments for relief from obligations |  | 75,000,000.0 |  |  |  |
Coal supply | PNM | San Juan Generating Station |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Other current assets | 26,300,000 | 23,400,000 | 26,300,000 |  |  |
Coal supply | NM Capital | San Juan Generating Station |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Requirement to post reclamation bonds |  | 118,700,000 |  |  |  |
Cash used to support bank letter or credit arrangement |  | 30,300,000 |  |  |  | $ 30,300,000
Increase in coal mine decommissioning liability | PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Regulatory disallowance, additional amount recorded |  |  |  | $ 800,000 |  |
Increase in coal mine decommissioning liability | PNM | Surface |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Regulatory disallowance, additional amount recorded | 3,600,000 |  |  |  |  |
Decrease in coal mine decommissioning liability | PNM | Underground |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Regulatory disallowance, additional amount recorded | $ 100,000 |  |  |  |  |
Decrease in coal mine decommissioning liability | PNM | Four Corners |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Regulatory disallowance, additional amount recorded |  |  | (2,500,000) |  |  |
Mine Reclamation Trust | PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Required contribution to reclamation trust, current fiscal year |  | 4,600,000 |  |  |  |
Reclamation trust funding, year 2 |  | 6,200,000 |  |  |  |
Reclamation trust funding, year 3 |  | 0 |  |  |  |
Four Corners CSA | PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Other commitment, due in first year |  |  |  |  | $ 2,000,000.0 |
Other commitment, due in second year |  | 2,100,000 |  |  |  |
Other commitment, due in third year |  | 2,100,000 |  |  |  |
Other commitment, due in fourth year |  | 2,100,000 |  |  |  |
Other commitment, due in fifth year |  | $ 2,100,000 |  |  |  |
San Juan Generating Station | Loss on long-term purchase commitment | PNM |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Annual funding post-term reclamation trust |  |  | $ 3,200,000 |  |  |

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