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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)
Mar. 31, 2016
USD ($)
Dec. 01, 2015
Allotment_Parcel
Jul. 13, 2015
a
Jan. 22, 2015
Allotment_Parcel
Apr. 02, 2014
Allotment_Parcel
Jan. 06, 2014
Allotment_Parcel
Aug. 31, 2013
May. 23, 2013
USD ($)
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]                  
Ownership percentage in nuclear reactor 10.20%                
Maximum potential assessment per incident $ 38,900,000                
Annual payment limitation related to incident 5,800,000                
Aggregate amount of all risk insurance 2,750,000,000.00                
Maximum amount under Nuclear Electric Insurance Limited 5,400,000                
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]                  
Liability insurance coverage 13,500,000,000.0                
Liability insurance coverage sublimit 2,250,000,000.00                
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Commercial Providers [Member] | Palo Verde Nuclear Generating Station [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Liability insurance coverage 375,000,000                
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Industry Wide Retrospective Assessment Program [Member] | Palo Verde Nuclear Generating Station [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Liability insurance coverage $ 13,100,000,000.0                
Continuous Highwall Mining [Member] | San Juan Generating Station [Member]                  
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%                
NMTRD Coal Severance Tax [Member] | Four Corners [Member]                  
Public Utilities, Commitments And Contingencies [Line Items]                  
Assessed coal severance surtax, penalty and interest               $ 30,000,000  
Share of settlement in addition to amounts previously paid $ 100,000                
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 (in landowners) | landowner                 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