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Commitments and Contingencies - Coal Supply (Details)
 1 Months Ended 12 Months Ended
Feb. 01, 2017
USD ($)
Feb. 01, 2016
USD ($)
Jan. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jul. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Feb. 21, 2017
USD ($)
Jul. 19, 2016
USD ($)
Jul. 07, 2016
payment
Jun. 30, 2016
Apr. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Other Commitments [Line Items]                            
Other current assets       $ 82,104,000   $ 73,444,000 $ 82,104,000              
Regulatory disallowances and restructuring costs           15,011,000 167,471,000 $ 1,062,000            
Funds contributed to mine reclamation trust         $ 1,900,000                  
San Juan Generating Station [Member] | Coal Supply [Member]                            
Other Commitments [Line Items]                            
Replacement reclamation bonds                   $ 118,700,000        
Public Service Company of New Mexico [Member]                            
Other Commitments [Line Items]                            
Other current assets       74,990,000   67,355,000 74,990,000              
Regulatory disallowances and restructuring costs           15,011,000 167,471,000 1,062,000            
Regulatory assets       342,910,000   368,855,000 342,910,000              
Public Service Company of New Mexico [Member] | Increase in coal mine decommissioning liability [Member]                            
Other Commitments [Line Items]                            
Net expense           4,500,000                
Public Service Company of New Mexico [Member] | Loss on long-term purchase commitment [Member] | San Juan Generating Station [Member]                            
Other Commitments [Line Items]                            
Annual funding post-term reclamation trust           7,000,000 4,300,000 $ 1,000,000            
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member]                            
Other Commitments [Line Items]                            
Reclamation trust funding, year 3           8,700,000                
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] | San Juan Generating Station [Member]                            
Other Commitments [Line Items]                            
Reclamation trust funding, next fiscal year           7,800,000                
Reclamation trust funding, year 2           8,300,000                
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] | Four Corners [Member]                            
Other Commitments [Line Items]                            
Reclamation trust funding, next fiscal year           2,000,000                
Reclamation trust funding, year 2           2,000,000                
Reclamation trust funding, year 3           2,100,000                
Number of annual installments (in payments) | payment                     13      
Public Service Company of New Mexico [Member] | Surface [Member]                            
Other Commitments [Line Items]                            
Regulatory assets           100,000,000                
Public Service Company of New Mexico [Member] | Surface [Member] | Loss on long-term purchase commitment [Member]                            
Other Commitments [Line Items]                            
Liability for interim storage costs       38,800,000   41,000,000 38,800,000              
Capped amount that can be collected from retail customers for final reclamation           100,000,000.0                
Public Service Company of New Mexico [Member] | Underground [Member] | Loss on long-term purchase commitment [Member]                            
Other Commitments [Line Items]                            
Liability for interim storage costs       11,400,000   14,000,000 11,400,000              
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Surface [Member] | Loss on long-term purchase commitment [Member]                            
Other Commitments [Line Items]                            
Regulatory disallowances and restructuring costs       16,500,000     16,500,000              
Estimate of possible loss           102,000,000                
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Underground [Member] | Loss on long-term purchase commitment [Member]                            
Other Commitments [Line Items]                            
Estimate of possible loss           127,800,000                
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Coal Supply [Member]                            
Other Commitments [Line Items]                            
Other current assets       $ 49,000,000   48,700,000 $ 49,000,000              
Estimated increase in coal cost                       48.00%    
NM Capital [Member] | BTMU Term Loan Agreement [Member]                            
Other Commitments [Line Items]                            
Long-term debt   $ 125,000,000.0       92,200,000                
NM Capital [Member] | BTMU Term Loan Agreement [Member] | Subsequent Event [Member]                            
Other Commitments [Line Items]                            
Long-term debt                 $ 82,800,000          
NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member]                            
Other Commitments [Line Items]                            
Funding provided   $ 125,000,000.0 $ 125,000,000.0                      
Variable interest rate   7.25%                        
Coal mine reclamation bonds to be posted with NMMMD                         $ 118,700,000 $ 161,600,000
Cash used to support bank letter or credit arrangement     30,300,000             $ 30,300,000     $ 40,000,000  
NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member] | Subsequent Event [Member] | Westmoreland San Juan [Member]                            
Other Commitments [Line Items]                            
Principal payment received $ 9,600,000                          
NM Capital [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member]                            
Other Commitments [Line Items]                            
Amount outstanding     $ 125,000,000     95,000,000                
Prepayment penalty           $ 0                
NM Capital [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member] | Subsequent Event [Member]                            
Other Commitments [Line Items]                            
Amount outstanding                 $ 85,400,000