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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)
 1 Months Ended 6 Months Ended
Mar. 28, 2017
Apr. 30, 2010
city
Jun. 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
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      
Palo Verde Nuclear Generating Station | PNM                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Number of cities to provide cooling water | city   5                  
Term of agreement for cooling water   40 years                  
Palo Verde Nuclear Generating Station | PNM | 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                
Palo Verde Nuclear Generating Station | PNM | Nuclear plant | Commercial providers                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Liability insurance coverage     450,000,000                
Palo Verde Nuclear Generating Station | PNM | Nuclear plant | Industry Wide Retrospective Assessment Program                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Liability insurance coverage     13,000,000,000.0                
Palo Verde Nuclear Generating Station | PNM | Nuclear plant | Maximum                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Liability insurance coverage     13,400,000,000.0                
Liability insurance coverage sublimit     $ 2,250,000,000.00                
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%                
Navajo Nation Allottee Matters | PNM                      
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