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Commitments and Contingencies - Coal Supply (Details) - USD ($)
 3 Months Ended
Feb. 01, 2016
Dec. 31, 2015
Mar. 31, 2016
Mar. 31, 2015
Apr. 28, 2016
Public Utilities, Commitments And Contingencies [Line Items]          
Other current assets   $ 82,104,000 $ 96,936,000    
Regulatory disallowances and restructuring costs     774,000 $ 215,000  
Public Service Company of New Mexico [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Other current assets   74,990,000 91,541,000    
Regulatory disallowances and restructuring costs     774,000 $ 215,000  
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Required contribution to Reclamation Trust, current fiscal year     4,000,000    
Required contribution to Reclamation Trust, next fiscal year     4,700,000    
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] | Four Corners [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Required contribution to Reclamation Trust, next fiscal year     1,900,000    
Anticipated funding to Reclamation Trust, next fiscal year     2,000,000    
Anticipated funding to Reclamation Trust, third fiscal year     2,100,000    
Public Service Company of New Mexico [Member] | Surface [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Regulatory assets     100,000,000    
Public Service Company of New Mexico [Member] | Loss on Long-term Purchase Commitment [Member] | Surface [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Loss contingency accrual   38,800,000 38,700,000    
Final reclamation, capped amount to be collected     100,000,000    
Public Service Company of New Mexico [Member] | Loss on Long-term Purchase Commitment [Member] | Underground [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Loss contingency accrual   11,400,000 11,700,000    
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Loss on Long-term Purchase Commitment [Member] | Surface [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Regulatory disallowances and restructuring costs   16,500,000      
Loss contingency, estimate of possible loss     97,600,000    
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Loss on Long-term Purchase Commitment [Member] | Underground [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Loss contingency, estimate of possible loss     118,900,000    
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Coal Supply [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Other current assets   $ 49,000,000 $ 65,200,000    
Estimated increase in coal cost     40.00%    
NM Capital [Member] | BTMU Term Loan Agreement [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Term loans $ 125,000,000.0        
NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Payments to fund long-term loans to unaffiliated third party     $ 125,000,000.0    
Loan agreement among several entities $ 125,000,000        
Initial interest rate 7.25%        
Requirement to post reclamation bonds $ 161,600,000        
Cash used to support bank letter or credit arrangement $ 40,000,000        
Number of days to provide security interest 180 days        
Subsequent Event [Member] | NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member]          
Public Utilities, Commitments And Contingencies [Line Items]          
Requirement to post reclamation bonds         $ 118,700,000