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Commitments and Contingencies - Coal Supply (Details) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Public Utilities, Commitments And Contingencies [Line Items]        
Other current assets $ 76,161 $ 58,471    
Public Service Company of New Mexico [Member] | Surface [Member]        
Public Utilities, Commitments And Contingencies [Line Items]        
Regulatory assets 100,000      
Public Service Company of New Mexico [Member] | Loss on Long-term Purchase Commitment [Member]        
Public Utilities, Commitments And Contingencies [Line Items]        
Public utilities, annual funding   1,000 $ 300 $ 3,500
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member]        
Public Utilities, Commitments And Contingencies [Line Items]        
Reclamation Trust Funding, current fiscal year 4,600      
Reclamation Trust Funding, next fiscal year 4,300      
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 24,700 25,700    
Final reclamation, capped amount to be collected 100,000      
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 9,300 8,600    
San Juan Generating Station [Member] | 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, estimate of possible loss 55,500      
San Juan Generating Station [Member] | 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, estimate of possible loss 93,300      
San Juan Generating Station [Member] | Coal Supply [Member] | Public Service Company of New Mexico [Member]        
Public Utilities, Commitments And Contingencies [Line Items]        
Other current assets $ 44,900 $ 37,300    
Public utilities, estimated increase in coal cost 30.00%