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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)
 1 Months Ended 3 Months Ended 9 Months Ended
Oct. 23, 2017
Allotment_Parcel
Mar. 28, 2017
Apr. 30, 2010
city
Sep. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
generating_unit
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% 8.00%                
Estimated underpaid surface mining royalties under proposed rate change       $ 5,000,000 $ 5,000,000                
PNM's share estimated underpaid surface mining royalties under proposed rate change       46.30% 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               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% 10.20%                
Number of units | generating_unit         3                
Maximum potential assessment per incident       $ 38,900,000.0 $ 38,900,000.0                
Annual payment limitation related to incident       5,800,000.0 5,800,000.0                
Aggregate amount of all risk insurance       2,750,000,000.00 2,750,000,000.00                
Maximum amount under Nuclear Electric Insurance Limited       5,400,000.0 5,400,000.0                
PNM | Maximum | Palo Verde Nuclear Generating Station | Nuclear plant                          
Public Utilities, Commitments And Contingencies [Line Items]                          
Liability insurance coverage       13,400,000,000.0 13,400,000,000.0                
Liability insurance coverage sublimit       2,250,000,000.00 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       450,000,000 450,000,000                
Industry Wide Retrospective Assessment Program | PNM | Palo Verde Nuclear Generating Station | Nuclear plant                          
Public Utilities, Commitments And Contingencies [Line Items]                          
Liability insurance coverage       $ 13,000,000,000.0 $ 13,000,000,000.0                
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                          
Public Utilities, Commitments And Contingencies [Line Items]                          
Unopposed motion filed for extension of procedural schedule 90 days                        
Subsequent event | PNM | Navajo Nation Allottee Matters                          
Public Utilities, Commitments And Contingencies [Line Items]                          
Number of allotments where landowners are revoking rights of way renewal consents (in allotment parcels) | Allotment_Parcel 2