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Commitments and Contingencies - Coal Supply (Details)
 3 Months Ended 6 Months Ended 12 Months Ended
Feb. 01, 2016
USD ($)
Jan. 31, 2016
USD ($)
Jun. 30, 2016
USD ($)
Jun. 30, 2015
USD ($)
Jun. 30, 2016
USD ($)
Jun. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Aug. 01, 2016
USD ($)
payment
Jul. 19, 2016
USD ($)
Jul. 07, 2016
USD ($)
Apr. 30, 2016
USD ($)
Public Utilities, Commitments And Contingencies [Line Items]                      
Other current assets     $ 103,483,000   $ 103,483,000   $ 82,104,000        
Regulatory disallowances and restructuring costs     0 $ 1,529,000 774,000 $ 1,744,000          
San Juan Generating Station [Member] | Coal Supply [Member] | Subsequent Event [Member]                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Replacement Coal Mine Reclamation Bonds posted by alternate surety companies                 $ 118,700,000    
Public Service Company of New Mexico [Member]                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Other current assets     96,625,000   96,625,000   74,990,000        
Regulatory disallowances and restructuring costs     0 $ 1,529,000 774,000 $ 1,744,000          
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     3,800,000   3,800,000            
Anticipated funding to Reclamation Trust, next fiscal year     4,700,000   4,700,000            
Anticipated funding to Reclamation Trust, third fiscal year     5,100,000   5,100,000            
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] | Four Corners [Member] | Subsequent Event [Member]                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Anticipated funding to Reclamation Trust, next fiscal year                   $ 2,000,000  
Anticipated funding to Reclamation Trust, third fiscal year                   2,000,000  
Number of annual installment payments | payment               13      
Required contribution to Reclamation Trust, next fiscal year                   $ 1,900,000  
Public Service Company of New Mexico [Member] | Surface [Member]                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Regulatory assets     100,000,000   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     38,200,000   38,200,000   38,800,000        
Final reclamation, capped amount to be collected         100,000,000.0            
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     12,000,000   12,000,000   11,400,000        
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     96,700,000   96,700,000            
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   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     $ 59,600,000   $ 59,600,000   49,000,000        
Estimated increase in coal cost     40.00%   40.00%            
NM Capital [Member] | BTMU Term Loan Agreement [Member]                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Term loans $ 125,000,000.0   $ 123,800,000.0   $ 123,800,000.0            
NM Capital [Member] | BTMU Term Loan Agreement [Member] | Subsequent Event [Member]                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Term loans               $ 107,800,000.0      
NM Capital [Member] | BTMU Term Loan Agreement [Member] | Maximum [Member]                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Debt-to-capital ratio covenant 65.00%                    
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       $ 118,700,000
Cash used to support bank letter or credit arrangement             $ 40,000,000        
NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member] | Subsequent Event [Member]                      
Public Utilities, Commitments And Contingencies [Line Items]                      
Cash used to support bank letter or credit arrangement                 $ 30,300,000