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Fair Value of Derivative and Other Financial Instruments - Narrative (Details)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) MW power_purchase_agreement gal	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) MWh MW	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) MWh gal
Debt Securities, Available-for-sale [Line Items]
Number of purchase power agreements \| power_purchase_agreement	3
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]
Number of purchase power agreements \| power_purchase_agreement	3
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	$ 600,000		$ 4,200,000	$ 1,200,000	$ (4,500,000)
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) \| MWh				111,512		0
Open gas commodity volume positions \| gal	0					0
PNM \| Commodity derivatives \| Designated as Hedging Instrument
Debt Securities, Available-for-sale [Line Items]
Derivative asset, gross liability	$ 700,000			$ 700,000
Derivative asset, gross asset	700,000			700,000
Other current assets	753,000			753,000		$ 1,096,000
Current derivative liability	2,703,000			2,703,000		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	100,000			100,000		0
Current derivative liability	$ 2,000,000			$ 2,000,000		0
PNM \| Portfolio One
Debt Securities, Available-for-sale [Line Items]
Number of megawatts available in purchase power agreement \| MW	85			85
PNM \| Portfolio Three
Debt Securities, Available-for-sale [Line Items]
Number of megawatts available in purchase power agreement \| MW	40			40
PNM \| Portfolio Two
Debt Securities, Available-for-sale [Line Items]
Number of megawatts available in purchase power agreement \| MW	150			150
PNM \| Tri-State
Debt Securities, Available-for-sale [Line Items]
Power to be sold to third party (in megawatts) \| MW	100			100
PNM \| Recurring
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities	$ 455,409,000			$ 455,409,000		440,115,000
Nuclear Decommissioning Trust \| PNM \| Recurring
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities	392,200,000			392,200,000		379,200,000
Mine Reclamation Trust \| PNM \| Recurring
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities	$ 63,200,000			$ 63,200,000		$ 60,900,000