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Fair Value of Derivative and Other Financial Instruments - Narrative (Details)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) MW	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) MMBTU MW	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) MMBTU
Debt Securities, Available-for-sale [Line Items]
Obligations to return cash	$ 900,000			$ 900,000		$ 900,000
Contract in a liability position	0			0		0
Debt securities, available-for-sale, unrealized loss position	$ 0			0		0
Other than temporary impairments				$ 0		0
PNM
Debt Securities, Available-for-sale [Line Items]
Expected exposure to market risk (in megawatts) | MW	65			65
Power to be sold to third party (in megawatts) | MW	36			36
Amounts recognized for right to reclaim cash	$ 0			$ 0		0
Cash collateral under margin arrangements	500,000			500,000		$ 500,000
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings	1,700,000		$ 4,000,000.0	$ 600,000	$ (8,700,000)
PNM | Natural Gas Cost | Unusual Weather
Debt Securities, Available-for-sale [Line Items]
Cost of energy		$ 20,000,000
PNM | Electricity, Purchased | Unusual Weather
Debt Securities, Available-for-sale [Line Items]
Cost of energy		$ 8,000,000
PNM | Commodity derivatives | Fair value hedging
Debt Securities, Available-for-sale [Line Items]
Economic Hedges (in mmbtu and mwh) | MMBTU				0		0
PNM | Commodity derivatives | Designated as Hedging Instrument
Debt Securities, Available-for-sale [Line Items]
Other current assets	1,008,000			$ 1,008,000		$ 1,096,000
PNM | Commodity derivatives | Designated as Hedging Instrument | Cost of energy
Debt Securities, Available-for-sale [Line Items]
Total gain	100,000		$ 100,000	100,000	$ 100,000
PNM | Fuel and purchased power costs | Commodity derivatives | Designated as Hedging Instrument
Debt Securities, Available-for-sale [Line Items]
Other current assets	$ 0			$ 0		0
PNM | Tri-State
Debt Securities, Available-for-sale [Line Items]
Power to be sold to third party (in megawatts) | MW	100			100
Recurring | PNM
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities	$ 453,007,000			$ 453,007,000		440,115,000
Nuclear Decommissioning Trust | Recurring | PNM
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities	391,900,000			391,900,000		379,200,000
Mine Reclamation Trust | Recurring | PNM
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities	$ 61,200,000			$ 61,200,000		$ 60,900,000

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