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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)
 1 Months Ended 9 Months Ended
Mar. 28, 2017
Sep. 30, 2012
landowner
Apr. 30, 2010
city
Sep. 30, 2018
USD ($)
generating_unit
Oct. 31, 2018
USD ($)
Dec. 01, 2015
Allotment_Parcel
Jul. 13, 2015
a
Jan. 22, 2015
Allotment_Parcel
Feb. 27, 2014
lawsuit
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%            
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                    
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  
Subsequent event | State and Local Jurisdiction | First Choice                    
Public Utilities, Commitments And Contingencies [Line Items]                    
Adjustment from settlement with taxing authority         $ 900,000