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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)
 1 Months Ended 3 Months Ended
Mar. 28, 2017
Apr. 30, 2010
city
Mar. 31, 2018
USD ($)
generating_unit
Oct. 23, 2017
Allotment_Parcel
Dec. 01, 2015
Allotment_Parcel
Jul. 13, 2015
a
Jan. 22, 2015
Allotment_Parcel
Apr. 02, 2014
landowner
Allotment_Parcel
Feb. 27, 2014
lawsuit
Jan. 06, 2014
Allotment_Parcel
Aug. 31, 2013
Sep. 30, 2012
landowner
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%                  
PNM | Navajo Nation Allottee Matters                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Number of landowners claiming to be Navajo allottees (in landowners) | landowner               43       43
Number of allotment parcels' appraisal requested for review (in allotment parcels) | Allotment_Parcel                   58    
Number of allotments where landowners are revoking rights of way renewal consents (in allotment parcels) | Allotment_Parcel       2     10 6        
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                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Number of cities to provide cooling water | city   5                    
Term of agreement for cooling water   40 years                    
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     $ 38,900,000.0                  
Annual payment limitation related to incident     5,800,000.0                  
Aggregate amount of all risk insurance     2,750,000,000.00                  
Maximum amount under Nuclear Electric Insurance Limited     5,400,000.0                  
PNM | Maximum | Palo Verde Nuclear Generating Station | Nuclear plant                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Liability insurance coverage     13,200,000,000.0                  
Liability insurance coverage sublimit     2,250,000,000.00                  
First Choice                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Additional tax due plus penalties and interest     5,000,000                  
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     $ 12,700,000,000.0                  
Pending litigation                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Number of pending claims | lawsuit                 2      
Written notification to terminate agreement, minimum period of time required 30 days                      
Settled litigation                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Number of pending claims | lawsuit                 1