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Commitments and Contingencies - The Clean Air Act (Details)
 3 Months Ended 6 Months Ended 12 Months Ended 49 Months Ended
Feb. 25, 2016
Dec. 16, 2015
USD ($)
MWh
MW
Jul. 31, 2015
USD ($)
MW
Mar. 02, 2015
lb / MMBTU
T
Mar. 31, 2017
USD ($)
lbsofnox / mmbtu
joint_owner
requirement
well
MW
Dec. 31, 2016
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Mar. 31, 2017
USD ($)
lbsofnox / mmbtu
joint_owner
requirement
well
MW
Dec. 31, 2017
portfolio
Jan. 26, 2016
state
Oct. 01, 2015
parts_per_billion
Sep. 30, 2015
parts_per_billion
May 14, 2015
lb / MMBTU
Dec. 30, 2013
Feb. 28, 2013
lawsuit
state
mine
Aug. 06, 2012
compliance_alternative
Jan. 31, 2010
parts_per_billion
Jul. 31, 2005
T
Dec. 31, 1999
state
Public Utilities, Commitments And Contingencies [Line Items]                                            
Regulatory disallowances and restructuring costs         $ 0   $ 774,000                              
Plant in service, held for future use, and to be abandoned         7,009,698,000 $ 6,944,534,000   $ 6,944,534,000 $ 6,944,534,000   $ 7,009,698,000                      
Accumulated depreciation and amortization         2,373,542,000 2,334,938,000   2,334,938,000 2,334,938,000   2,373,542,000                      
Public Service Company of New Mexico                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Regulatory disallowances and restructuring costs         0   774,000                              
Plant in service, held for future use, and to be abandoned         5,383,068,000 5,359,211,000   5,359,211,000 5,359,211,000   5,383,068,000                      
Accumulated depreciation and amortization         $ 1,832,695,000 1,809,528,000   1,809,528,000 1,809,528,000   $ 1,832,695,000                      
Public Service Company of New Mexico | Reeves Station                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Requirements to be implemented | requirement         0           0                      
Public Service Company of New Mexico | Forecast                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of resource portfolios | portfolio                       2                    
Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Plant in service, held for future use, and to be abandoned         $ 471,800,000           $ 471,800,000                      
Accumulated depreciation and amortization         206,400,000           206,400,000                      
Net book value         265,400,000           265,400,000                      
Public Service Company of New Mexico | San Juan Generating Station | Loss on long-term purchase commitment | Surface                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Regulatory disallowances and restructuring costs                   $ 16,500,000                        
Estimate of possible loss         $ 102,000,000           $ 102,000,000                      
Public Service Company of New Mexico | San Juan Generating Station | Forecast                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of portfolios for continuing operations | portfolio                       1                    
Number of portfolios with discontinued operations | portfolio                       1                    
Public Service Company of New Mexico | San Juan Generating Station and Four Corners | Maximum                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Proposed government standard emission limit (in ozone parts per million) | parts_per_billion                                       70    
Government standard emission limit (in ozone parts per million) | parts_per_billion                           70 75              
Public Service Company of New Mexico | San Juan Generating Station and Four Corners | Minimum                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Proposed government standard emission limit (in ozone parts per million) | parts_per_billion                                       60    
PNMR and PNM | Palo Verde Nuclear Generating Station Unit 3                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of megawatts | MW         134           134                      
PNMR Development | Energy equipment                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Costs of installing equipment     $ 7,600,000                                      
Clean Air Act related to regional haze                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of states to address regional haze (in states) | state                                           50
Potential to emit tons per year of visibility impairing pollution (in tons, more than) | T                                         250  
Clean Air Act, SNCR | San Juan Generating Station Unit 4                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of other joint owners | joint_owner         8           8                      
Clean Air Act, SNCR | Public Service Company of New Mexico                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Estimated costs to remain unrecovered   $ 20,000,000                                        
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Units 1 and 4 | Plan                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Ownership percentage     66.30%                                      
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Units 1 and 4 | Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other Costs                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Total costs for equipment                     $ 77,900,000                      
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Current ownership interest (in megawatts) | MW   418                                        
Time period to recover retired units' remaining net book value   20 years                                        
Recovery percentage of estimated undepreciated value at 12/31/17   50.00%     50.00%         50.00% 50.00%                      
Regulatory disallowances and restructuring costs                   $ 127,600,000                        
Expense reflected in regulatory disallowances and restructuring costs             $ 800,000   3,700,000                          
Accumulated plant write-off, disallowance         $ 128,600,000           $ 128,600,000                      
Net carrying value of utility plant         $ 136,800,000           $ 136,800,000                      
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3 | Increase in coal mine decommissioning liability                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Regulatory disallowances and restructuring costs                   $ 165,700,000                        
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 4                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Additional ownership to be obtained (in megawatts) | MW   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%           38.80%                      
Ownership percentage         38.50%           38.50%                      
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 4 | Maximum                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Annual cost of Renewable Energy Credits   $ 7,000,000                                        
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 4 | Plan                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Ownership percentage     77.30%                                      
Clean Air Act, SNCR | Public Service Company of New Mexico | Palo Verde Nuclear Generating Station Unit 3                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Estimated undepreciated value at 12/31/17   $ 156,000,000                                        
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 3                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Percentage of ownership held by exiting owners         50.00%           50.00%                      
Ownership percentage         50.00%           50.00%                      
Clean Air Act, SNCR | PNMR and PNM | Palo Verde Nuclear Generating Station Unit 3                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of megawatts | MW   134                                        
Clean Air Act, SNCR | PNMR Development | San Juan Generating Station Unit 4                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Additional ownership to be obtained (in megawatts) | MW     65                                      
Expense for cost, obligation to reimburse           $ 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, Commitments And Contingencies [Line Items]                                            
Additional ownership to be obtained (in megawatts) | MW   132                                        
Clean Air Act, Post-2022 coal supply future rate case | Public Service Company of New Mexico | San Juan Generating Station Unit 4                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Period of time agreed to by all parties to settle rate case   6 months                                        
Increase in coal mine decommissioning liability | Public Service Company of New Mexico                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Expense reflected in regulatory disallowances and restructuring costs               $ 4,500,000 $ 4,500,000                          
Clean Air Act related to post combustion controls | Public Service Company of New Mexico | Four Corners                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of compliance alternatives | compliance_alternative                                     2      
Plant requirement to meet NOx emissions limit (in pounds of NOx per MMBTU) | lbsofnox / mmbtu         0.015           0.015                      
Plant requirement to meet opacity limit         20.00%           20.00%                      
Rule imposes opacity limitation on certain fugitive dust emissions from coal and material handling operations         20.00%           20.00%                      
Estimate of possible loss         $ 90,200,000           $ 90,200,000                      
Clean Air Act related to post combustion controls | Public Service Company of New Mexico | Four Corners Units 4 and 5 (Coal)                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Ownership percentage         13.00%           13.00%                      
Clean Air Act related to post combustion controls | Public Service Company of New Mexico | Four Corners Units 1, 2 and 3 (Coal)                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Ownership percentage                                 0.00%          
Clean Power Plan                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Period of potential extension to meet environmental targets         2 years                                  
Number of states that filed a petition against the Clean Power Plan | state                         29                  
National Ambient Air Quality Standards, 2015 EPA Legal Settlement | Public Service Company of New Mexico                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Period of time to act on settlement       16 months                                    
Emissions tons of SO2 per year (more than) | T       16,000                                    
National Ambient Air Quality Standards, 2015 EPA Legal Settlement | Public Service Company of New Mexico | Minimum                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Emissions tons of SO2 per year (more than) | T       2,600                                    
One-hour SO2 emissions rate (in pounds per MMBTU) | lb / MMBTU       0.45                                    
National Ambient Air Quality Standards | Public Service Company of New Mexico | San Juan Generating Station                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Revised SO2 emissions (in pounds per MMBTU) | lb / MMBTU                               0.10            
National Ambient Air Quality Standards, 2015 EPA Legal Settlement | Public Service Company of New Mexico                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Period of time from state designation, to provide implementation plans 36 months                                          
WEG v OSM lawsuit | Public Service Company of New Mexico                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of mines impacted | mine                                   7        
Number of states impacted | state                                   4        
Number of claims for relief, filed (in lawsuits) | lawsuit                                   15        
WEG v OSM lawsuit | Public Service Company of New Mexico | San Juan Generating Station                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of claims for relief, filed (in lawsuits) | lawsuit                                   2        
Santa Fe generating station | Public Service Company of New Mexico                                            
Public Utilities, Commitments And Contingencies [Line Items]                                            
Number of wells with elevated levels of nitrates | well         3           3