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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)
 3 Months Ended
Sep. 30, 2015
USD ($)
Jul. 13, 2015
a
Jan. 22, 2015
Allotment_Parcel
Apr. 02, 2014
Allotment_Parcel
Apr. 01, 2014
USD ($)
Jan. 06, 2014
Allotment_Parcel
Aug. 31, 2013
May. 23, 2013
USD ($)
Sep. 30, 2012
landowner
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Public utilities, ownership percentage in nuclear reactor 10.20%                
Public utilities, maximum potential assessment per incident $ 38,900,000                
Public utilities, annual payment limitation related to incident 5,700,000                
Public utilities, aggregate amount of all risk insurance 2,750,000,000                
Public utilities, maximum amount under Nuclear Electric Insurance Limited 5,400,000.0                
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] | Maximum [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Public utilities, liability insurance coverage 13,400,000,000.0                
Public utilities, liability insurance coverage sublimit         $ 2,250,000,000        
Public Service Company of New Mexico [Member] | Commercial Providers [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Public utilities, liability insurance coverage 375,000,000                
Public Service Company of New Mexico [Member] | Industry Wide Retrospective Assessment Program [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Public utilities, liability insurance coverage $ 13,000,000,000.0                
Continuous Highwall Mining [Member] | San Juan Generating Station [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Public utilities, proposed retroactive surface mining royalty rate             12.50%    
Public utilities, current surface mining royalty rate applied between 2000 and 2003 8.00%                
Public utilities, estimated underpaid surface mining royalties under proposed rate change $ 5,000,000                
Public utilities, PNM share estimated underpaid surface mining royalties under proposed rate change 46.30%                
NMTRD Coal Severance Tax [Member] | Four Corners [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Public utilities, assessed coal severance surtax, penalty and interest               $ 30,000,000  
NMTRD Coal Severance Tax [Member] | Public Service Company of New Mexico [Member] | Four Corners [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Public utilities, PNM share, assessed coal severance surtax penalty and interest to pass through FFPAC               9.40%  
SPS Complaint, Federal Power Act [Member] | Public Service Company of New Mexico [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Public Utilities, Proposed settlement pending regulatory approval $ 4,200,000                
Public Utilities, Portion of proposed settlement to be credited to customers, pending regulatory approval $ 2,600,000                
Navajo Nation Allottee Matters [Member] | Public Service Company of New Mexico [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Number of landowners claiming to be Navajo allottees (landowners) | landowner                 43
Number of allotment parcels' appraisal requested for review (allotment parcels) | Allotment_Parcel           58      
Number of allotments where landowners are revoking rights of way renewal consents (allotment parcels) | Allotment_Parcel     10 6          
Area of land (acres) | a   15.49