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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)
 1 Months Ended 3 Months Ended
Sep. 30, 2012
landowner
Mar. 31, 2021
USD ($)
lawsuit
plaintiff
generating_unit
shareholder
Dec. 01, 2015
Allotment_Parcel
Jul. 13, 2015
a
Jan. 22, 2015
Allotment_Parcel
Aug. 31, 2013
Continuous highwall mining | San Juan Generating Station            
Public Utilities, Commitments And Contingencies [Line Items]            
Proposed retroactive surface mining royalty rate           12.50%
Surface mining royalty rate applied   8.00%        
Estimated underpaid surface mining royalties under proposed rate change   $ 5,000,000        
PNM's share estimated underpaid surface mining royalties under proposed rate change   46.30%        
Merger-Related Litigation            
Public Utilities, Commitments And Contingencies [Line Items]            
Number of shareholders involved in lawsuit | shareholder   6        
Number of lawsuits for the Southern District of New York | lawsuit   5        
Number of lawsuits filed for the Eastern District of New York | lawsuit   1        
Number of plaintiffs who filed notices of voluntary dismissal | plaintiff   5        
PNM | Navajo Nation Allottee Matters            
Public Utilities, Commitments And Contingencies [Line Items]            
Number of landowners involved in the appeal | landowner 43          
Number of allotments where landowners are revoking rights of way renewal consents (in allotment parcels) | Allotment_Parcel     2   10  
Area of land (in acres) | a       15.49    
Number of allotment parcels at issue that are not to be condemned | Allotment_Parcel     2      
Number of allotment parcels at issue | Allotment_Parcel     5      
PNM | Palo Verde Nuclear Generating Station | Nuclear plant            
Public Utilities, Commitments And Contingencies [Line Items]            
Ownership percentage in nuclear reactor   10.20%        
Number of units | generating_unit   3        
Maximum potential assessment per incident   $ 42,100,000        
Annual payment limitation related to incident   6,200,000        
Aggregate amount of all risk insurance   2,800,000,000        
Maximum amount under Nuclear Electric Insurance Limited   5,400,000        
PNM | Maximum | Palo Verde Nuclear Generating Station | Nuclear plant            
Public Utilities, Commitments And Contingencies [Line Items]            
Liability insurance coverage   13,700,000,000        
Liability insurance coverage sublimit   2,250,000,000        
Commercial providers | PNM | Palo Verde Nuclear Generating Station | Nuclear plant            
Public Utilities, Commitments And Contingencies [Line Items]            
Liability insurance coverage   450,000,000        
Industry Wide Retrospective Assessment Program | PNM | Palo Verde Nuclear Generating Station | Nuclear plant            
Public Utilities, Commitments And Contingencies [Line Items]            
Liability insurance coverage   $ 13,200,000,000