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Commitments and Contingencies - Coal Supply (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Jan. 31, 2016
PNM | Loss on long-term purchase commitment | Surface |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Loss contingency accrual | $ 44.5 | $ 50.0 |  |
PNM | Loss on long-term purchase commitment | Underground |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Loss contingency accrual | 47.9 | 26.2 |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Estimate of possible loss | 51.4 |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Estimate of possible loss | 59.7 |  |  |
Coal supply | San Juan Generating Station |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Cash used to support bank letter or credit arrangement |  |  |  | $ 30.3
Coal supply | NM Capital | San Juan Generating Station |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Requirement to post reclamation bonds | 118.7 |  |  |
Cash used to support bank letter or credit arrangement | 30.3 |  |  |
Four Corners CSA | PNM |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Funds contributed, mine reclamation trust |  | 0.2 | $ 2.4 |
Reclamation trust funding, remainder of year | 3.1 |  |  |
Reclamation trust funding, 2025 | 1.3 |  |  |
Reclamation trust funding, year 2026 | 1.5 |  |  |
San Juan Generating Station |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Reclamation costs | 20.9 |  |  |
Regulatory disallowance | 4.5 |  |  |
San Juan Generating Station | Surface |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Increase in regulatory liabilities | 4.0 |  |  |
Decrease in deferred regulatory assets | 0.5 |  |  |
Reclamation, maximum collection from customers | 100.0 |  |  |
San Juan Generating Station | Underground |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Increase in regulatory liabilities | 17.0 |  |  |
San Juan Generating Station | Loss on long-term purchase commitment | PNM |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Annual funding post-term reclamation trust |  | $ 2.7 |  |
Annual funding post-term reclamation trust, remainder of year | 20.7 |  |  |
Annual funding post-term reclamation trust, year one | 3.9 |  |  |
Annual funding post-term reclamation trust, year two | 4.0 |  |  |
San Juan Generating Station and Four Corners | Mine Reclamation Trust | PNM |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Customer reclamation funding cost | $ 100.0 |  |  |

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