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Commitments and Contingencies - Coal Supply (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2024	Dec. 31, 2023	Jan. 31, 2016
PNM | Four Corners CSA
Public Utilities, Commitments And Contingencies [Line Items]
Funds contributed, mine reclamation trust			$ 3.2	$ 0.2
Reclamation trust funding, 2025			2.1
Reclamation trust funding, year 2026			2.9
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		118.7
Cash used to support bank letter or credit arrangement	30.3		30.3
Underground | PNM | Loss on long-term purchase commitment
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual	54.5		54.5	26.2
Underground | PNM | San Juan Generating Station | Loss on long-term purchase commitment
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss	65.0		65.0
Surface | PNM | Loss on long-term purchase commitment
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual	42.2		42.2	50.0
Surface | PNM | San Juan Generating Station | Loss on long-term purchase commitment
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss	48.7		48.7
San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Reclamation costs	12.1	$ 20.9
Regulatory disallowance		4.0
Regulatory disallowance related to other costs		0.5
San Juan Generating Station | PNM | Loss on long-term purchase commitment
Public Utilities, Commitments And Contingencies [Line Items]
Annual funding post-term reclamation trust				$ 2.7
Annual funding post-term reclamation trust, remainder of year			25.1
Annual funding post-term reclamation trust, year one			2.4
Annual funding post-term reclamation trust, year two			0.6
San Juan Generating Station | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Increase in regulatory liabilities	6.0	17.0
San Juan Generating Station | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Increase in regulatory liabilities	$ 6.1
Reclamation, maximum collection from customers		$ 100.0
San Juan Generating Station and Four Corners | PNM | Mine Reclamation Trust
Public Utilities, Commitments And Contingencies [Line Items]
Customer reclamation funding cost			$ 100.0

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