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Commitments and Contingencies - Coal Supply (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Jan. 31, 2016
Other Commitments [Line Items] |  |  |  |  |
Coal requirement, minimum required payment, year one |  | $ 42.5 |  |  |
Coal requirement, minimum required payment, year two |  | 85.9 |  |  |
Public Utilities, Coal Requirement, Coal requirement, minimum required payment, year three |  | 85.9 |  |  |
Coal requirement, minimum required payment, year four |  | 89.8 |  |  |
Coal requirement, minimum required payment, year five |  | 89.8 |  |  |
Coal requirement, minimum required payment, after year five |  | 71.0 |  |  |
San Juan Generating Station |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Reclamation and mine shutdown provision | $ 20.9 | 12.1 |  |  |
Regulatory disallowance | 4.0 |  |  |  |
Other costs | 0.5 |  |  |  |
Underground | San Juan Generating Station |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Regulatory liabilities | $ 17.0 | 6.0 |  |  |
Surface | San Juan Generating Station |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Reclamation and mine shutdown provision |  | 1.6 |  |  |
Regulatory liabilities |  | 6.1 |  |  |
Reclamation, maximum collection from customers |  | 100.0 |  |  |
San Juan Generating Station | Coal Supply |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Issuance in letters of credit |  |  |  |  | $ 30.3
NM Capital | San Juan Generating Station | Coal Supply |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Coal mine reclamation bonds to be posted with NMMMD |  | 118.7 |  |  |
Issuance in letters of credit |  | 30.3 |  |  |
PNM | Loss on Long-Term Purchase Commitment | San Juan Generating Station |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Annual funding post-term reclamation trust |  | 27.3 | $ 2.7 | $ 10.0 |
PNM | Loss on Long-Term Purchase Commitment | San Juan Generating Station |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Public utilities, annual funding post-term reclamation trust, year one |  | 1.0 |  |  |
Public utilities, annual funding post-term reclamation trust, year two |  | 0.6 |  |  |
PNM | Four Corners CSA |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Funds contributed to mine reclamation trust |  | 3.2 | 0.2 | $ 2.4 |
Reclamation trust funding, year one |  | 0.5 |  |  |
Reclamation trust funding, year two |  | 0.5 |  |  |
PNM | Mine Reclamation Trust | San Juan Generating Station and Four Corners |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Customer reclamation funding cost, maximum |  | 100.0 |  |  |
PNM | Underground | Loss on Long-Term Purchase Commitment |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Liability for interim storage costs |  | 52.9 | 26.2 |  |
PNM | Surface | Loss on Long-Term Purchase Commitment |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Liability for interim storage costs |  | 38.3 | $ 50.0 |  |
PNM | San Juan Generating Station | Underground | Loss on Long-Term Purchase Commitment |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Estimate of possible loss |  | 62.7 |  |  |
PNM | San Juan Generating Station | Surface | Loss on Long-Term Purchase Commitment |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Estimate of possible loss |  | $ 51.1 |  |  |

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