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Fair Value of Derivative and Other Financial Instruments - Narrative (Details)
3 Months Ended 4 Months Ended
Apr. 23, 2024
MW
Mar. 31, 2024
USD ($)
Mar. 31, 2023
USD ($)
Sep. 30, 2023
MW
Dec. 31, 2023
USD ($)
Debt Securities, Available-for-sale [Line Items]          
Obligations to return cash         $ 200,000
PNM          
Debt Securities, Available-for-sale [Line Items]          
Amounts recognized for right to reclaim cash   $ 0     0
Cash collateral under margin arrangements   300,000     200,000
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings   13,400,000 $ 2,000,000    
PNM | Commodity derivatives | Designated as Hedging Instrument          
Debt Securities, Available-for-sale [Line Items]          
Other current assets   835,000     826,000
Current derivative liability   0     0
PNM | Portfolio One          
Debt Securities, Available-for-sale [Line Items]          
Number of megawatts available in purchase power agreement (in megawatts) | MW       85  
PNM | Portfolio One | Subsequent event          
Debt Securities, Available-for-sale [Line Items]          
Number of megawatts available in purchase power agreement (in megawatts) | MW 150        
PNM | Portfolio Two          
Debt Securities, Available-for-sale [Line Items]          
Number of megawatts available in purchase power agreement (in megawatts) | MW       50  
PNM | Portfolio Two | Subsequent event          
Debt Securities, Available-for-sale [Line Items]          
Number of megawatts available in purchase power agreement (in megawatts) | MW 100        
PNM | Portfolio Three          
Debt Securities, Available-for-sale [Line Items]          
Number of megawatts available in purchase power agreement (in megawatts) | MW       50  
PNM | Fuel and purchased power costs | Commodity derivatives | Designated as Hedging Instrument          
Debt Securities, Available-for-sale [Line Items]          
Other current assets         800,000
Current derivative liability         0
PNM | Recurring          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale debt securities   448,164,000     444,408,000
Nuclear Decommissioning Trust | PNM | Recurring          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale debt securities   371,000,000     361,000,000
SJGS Decommissioning Trust | PNM | Recurring          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale debt securities   11,700,000     12,300,000
Mine Reclamation Trust | PNM | Recurring          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale debt securities   $ 65,500,000     $ 71,100,000