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Commitments and Contingencies - The Clean Air Act (Details)
 1 Months Ended 12 Months Ended 35 Months Ended
Dec. 16, 2015
USD ($)
MWh
MW
Jul. 17, 2015
Dec. 20, 2013
USD ($)
$ / kw
MW
Aug. 31, 2011
generating_unit
Dec. 31, 2018
USD ($)
lb / MMBTU
generating_unit
joint_owner
MW
Dec. 31, 2017
USD ($)
MW
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jan. 31, 2016
USD ($)
Jan. 26, 2016
state
Jul. 31, 2015
MW
Aug. 06, 2012
compliance_alternative
Jul. 31, 2005
T
Dec. 31, 1999
state
Public Utilities, General Disclosures [Line Items]                            
Plant in service and held for future use         $ 7,548,581,000 $ 7,238,285,000                
Accumulated depreciation and amortization         2,604,177,000 2,592,692,000                
Regulatory disallowances and restructuring costs         65,598,000 27,036,000 $ 15,011,000              
Regulatory assets         $ 598,930,000 600,672,000                
San Juan Generating Station                            
Public Utilities, General Disclosures [Line Items]                            
Number of other entities (in joint owner) | joint_owner         5                  
Four Corners                            
Public Utilities, General Disclosures [Line Items]                            
Approved lease extension term   25 years                        
Public Service Company of New Mexico                            
Public Utilities, General Disclosures [Line Items]                            
Plant in service and held for future use         $ 5,623,520,000 5,501,070,000                
Accumulated depreciation and amortization         2,006,266,000 2,029,534,000                
Regulatory disallowances and restructuring costs         66,339,000 27,036,000 $ 15,011,000              
Regulatory assets         $ 460,903,000 $ 459,239,000                
Power to be sold to third party (in megawatts) | MW         36 36                
Public Service Company of New Mexico | San Juan Generating Station | Surface | Loss on long-term purchase commitment                            
Public Utilities, General Disclosures [Line Items]                            
Regulatory disallowances and restructuring costs               $ 16,500,000            
Estimate of possible loss         $ 103,200,000                  
Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3                            
Public Utilities, General Disclosures [Line Items]                            
Recovery percentage of estimated undepreciated value         50.00%                  
Plant in service and held for future use         $ 439,400,000                  
Accumulated depreciation and amortization         188,300,000                  
Net book value         251,100,000                  
Regulatory assets         125,500,000                  
Reversal of plant write-off         3,000,000                  
Reversal of recorded loss for other unrecoverable costs         1,000,000                  
Total reversed losses         4,000,000                  
Public Service Company of New Mexico | San Juan Generating Station Unit 4                            
Public Utilities, General Disclosures [Line Items]                            
Pre-tax impairment of investments         $ 121,800,000                  
Public Service Company of New Mexico | Four Corners                            
Public Utilities, General Disclosures [Line Items]                            
Number of units (in generating units) | generating_unit         2                  
Public Service Company of New Mexico | Four Corners Units 4 and 5 (Coal)                            
Public Utilities, General Disclosures [Line Items]                            
Plant in service and held for future use         $ 276,960,000                  
Accumulated depreciation and amortization         98,085,000                  
PNMR Development | Energy Equipment                            
Public Utilities, General Disclosures [Line Items]                            
Cost of installing equipment         $ 7,600,000                  
Environmental Protection Agency | Public Service Company of New Mexico | San Juan Generating Station                            
Public Utilities, General Disclosures [Line Items]                            
Number of units (in generating units) | generating_unit       4                    
Clean Air Act related to Regional Haze                            
Public Utilities, General Disclosures [Line Items]                            
Number of states to address regional haze | state                           50
Potential to emit tons per year of visibility impairing pollution, maximum (in tons) | T                         250  
Clean Air Act, SNCR | San Juan Generating Station Unit 4                            
Public Utilities, General Disclosures [Line Items]                            
Number of other entities (in joint owner) | joint_owner         8                  
Clean Air Act, SNCR | San Juan Generating Station Unit 4 | CALIFORNIA                            
Public Utilities, General Disclosures [Line Items]                            
Number of other entities (in joint owner) | joint_owner         3                  
Clean Air Act, SNCR | Public Service Company of New Mexico                            
Public Utilities, General Disclosures [Line Items]                            
Estimated costs to remain unrecovered $ 20,000,000                          
Regulatory disallowances and restructuring costs               $ 21,600,000            
Net expense           $ 3,700,000                
Revision of net book value           900,000                
Reclamation expense           4,500,000                
Clean Air Act, SNCR | Public Service Company of New Mexico | Restructuring costs                            
Public Utilities, General Disclosures [Line Items]                            
Net expense           $ 1,700,000                
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station                            
Public Utilities, General Disclosures [Line Items]                            
Overall reduction of ownership (in megawatts) | MW     418                      
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3                            
Public Utilities, General Disclosures [Line Items]                            
Requested time period to recover retired units NBV 20 years   20 years                      
Newly identified replacement solar generation (in megawatts) | MW     40                      
Current ownership (in megawatts) | MW 418                          
Recovery percentage of estimated undepreciated value 50.00%             50.00%            
Pre tax losses aggregated               $ 165,700,000            
Regulatory disallowances and restructuring costs               $ 127,600,000            
Accumulated plant write-off, disallowance         $ 128,600,000                  
Clean Air Act, SNCR | Public Service Company of New Mexico | Palo Verde Nuclear Generating Station Unit 3                            
Public Utilities, General Disclosures [Line Items]                            
Number of megawatts | MW           134                
Estimated undepreciated value $ 154,900,000                          
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 4                            
Public Utilities, General Disclosures [Line Items]                            
Additional ownership to be obtained (in megawatts) | MW 65       65           132      
Estimated rate base value $ 0                          
Coal-fired generation (in megawatts) | MW 197                          
Number of megawatt hours of renewable energy certificates to be acquired and retired (in megawatt hours) | MWh 1                          
Percentage of ownership held by exiting owners         38.80%                  
Ownership percentage         38.50%                  
Potential acquisition of ownership (in megawatts) | MW         65                  
Estimated increase in cost of acquired interest due to plant in service and accumulated depreciation         $ 261,800,000                  
Pre-tax impairment of investments         35,000,000                  
Undepreciated investment in ownership to be obtained         11,900,000                  
Forecasted undepreciated investment         23,100,000                  
Remaining undepreciated investment         $ 373,600,000                  
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 3                            
Public Utilities, General Disclosures [Line Items]                            
Percentage of ownership held by exiting owners         50.00%                  
Ownership percentage         50.00%                  
Clean Air Act, SNCR | Public Service Company of New Mexico | Maximum | San Juan Generating Station Unit 4                            
Public Utilities, General Disclosures [Line Items]                            
Annual cost of renewable energy credits, maximum $ 7,000,000.0                          
Clean Air Act, SNCR | Public Service Company of New Mexico | Installation costs including construction management, gross receipts taxes, AFUDC, and other PNM costs | San Juan Generating Station Units 1 and 4                            
Public Utilities, General Disclosures [Line Items]                            
Portion of costs for SNCRs and BDT equipment                 $ 77,700,000          
Clean Air Act, SNCR | Public Service Company of New Mexico | Installation costs including construction management, gross receipts taxes, AFUDC, and other PNM costs | Maximum | San Juan Generating Station Units 1 and 4                            
Public Utilities, General Disclosures [Line Items]                            
Estimated installation capital costs     $ 82,000,000                      
Clean Air Act, SNCR | Public Service Company of New Mexico | Scenario, plan | San Juan Generating Station Units 1 and 4                            
Public Utilities, General Disclosures [Line Items]                            
Ownership percentage         66.30%                  
Clean Air Act, SNCR | Public Service Company of New Mexico | Scenario, plan | San Juan Generating Station Unit 4                            
Public Utilities, General Disclosures [Line Items]                            
Ownership percentage         77.30%                  
Clean Air Act, SNCR | PNMR and PNM | Palo Verde Nuclear Generating Station Unit 3                            
Public Utilities, General Disclosures [Line Items]                            
Number of megawatts | MW 134   134                      
Proposed value per kilowatt | $ / kw     2,500                      
Clean Air Act, SNCR | PNMR Development | San Juan Generating Station Unit 4                            
Public Utilities, General Disclosures [Line Items]                            
Reimbursement amount           $ 600,000                
Potential acquisition of ownership (in megawatts) | MW                     65      
Clean Air Act, SNCR Hearing Examiner Recommended Denial | Public Service Company of New Mexico | San Juan Generating Station Unit 4                            
Public Utilities, General Disclosures [Line Items]                            
Additional ownership to be obtained (in megawatts) | MW 132       132                  
Estimated undepreciated value $ 20,700,000.0                          
Clean Air Act Related to Post Combustion Controls | Public Service Company of New Mexico | Four Corners                            
Public Utilities, General Disclosures [Line Items]                            
Number of compliance alternatives | compliance_alternative                       2    
Government standard emissions limit (in pounds per MMBTU) | lb / MMBTU         0.015                  
Plant requirement to meet opacity limit         20.00%                  
Rule imposes opacity limitation on certain fugitive dust emissions from coal and material handling operations         20.00%                  
Estimate of possible loss         $ 88,700,000                  
Clean Air Act Related to Post Combustion Controls | Public Service Company of New Mexico | Four Corners Units 4 and 5 (Coal)                            
Public Utilities, General Disclosures [Line Items]                            
Ownership percentage         13.00%                  
Clean Power Plan                            
Public Utilities, General Disclosures [Line Items]                            
Number of states that filed a petition against the Clean Power Plan | state                   29