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Commitments and Contingencies - The Clean Air Act (Details)
 3 Months Ended 9 Months Ended 12 Months Ended 43 Months Ended
Feb. 25, 2016
Dec. 16, 2015
USD ($)
MWh
MW
Mar. 02, 2015
lb / MMBTU
T
Sep. 30, 2016
USD ($)
joint_owner
requirement
well
MW
Mar. 31, 2016
USD ($)
Sep. 30, 2015
USD ($)
parts_per_billion
Sep. 30, 2016
USD ($)
joint_owner
requirement
well
MW
Sep. 30, 2015
USD ($)
parts_per_billion
Dec. 31, 2015
USD ($)
Sep. 30, 2016
USD ($)
joint_owner
requirement
well
MW
Feb. 09, 2016
Jan. 26, 2016
state
Oct. 01, 2015
parts_per_billion
Jul. 31, 2015
MW
Dec. 30, 2013
lbsofnox / mmbtu
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       $ 16,451,000   $ 0 $ 17,225,000 $ 1,744,000                        
Plant in service, held for future use, and to be abandoned       6,842,017,000     6,842,017,000   $ 6,307,261,000 $ 6,842,017,000                    
Accumulated depreciation and amortization       2,312,560,000     2,312,560,000   2,058,772,000 2,312,560,000                    
Public Service Company of New Mexico [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Regulatory disallowances and restructuring costs       16,451,000   $ 0 17,225,000 $ 1,744,000                        
Plant in service, held for future use, and to be abandoned       5,303,910,000     5,303,910,000   4,833,303,000 5,303,910,000                    
Accumulated depreciation and amortization       $ 1,785,507,000     $ 1,785,507,000   1,569,549,000 $ 1,785,507,000                    
Public Service Company of New Mexico [Member] | Reeves station [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Requirements to be implemented | requirement       0     0     0                    
Public Service Company of New Mexico [Member] | San Juan Generating Station Units 2 and 3 [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Plant in service, held for future use, and to be abandoned       $ 471,600,000     $ 471,600,000     $ 471,600,000                    
Accumulated depreciation and amortization       201,400,000     201,400,000     201,400,000                    
Net book value       270,200,000     270,200,000     270,200,000                    
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                      
Estimate of possible loss       $ 103,000,000     $ 103,000,000     $ 103,000,000                    
Public Service Company of New Mexico [Member] | San Juan Generating Station And Four Corners [Member] | Maximum [Member]                                        
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           75   75         70              
Public Service Company of New Mexico [Member] | San Juan Generating Station And Four Corners [Member] | Minimum [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Proposed government standard emission limit (in ozone parts per million) | parts_per_billion                                   60    
PNMR and PNM [Member] | Palo Verde Nuclear Generating Station Unit 3 [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Number of megawatts | MW       134     134     134                    
Clean Air Act related to Regional Haze [Member]                                        
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 [Member] | San Juan Generating Station Unit 4 [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Number of other joint owners | joint_owner       8     8     8                    
Clean Air Act, SNCR [Member] | Public Service Company of New Mexico [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Estimated costs to remain unrecovered   $ 20,000,000                                    
Expense reflected in regulatory disallowances and restructuring costs       $ 5,200,000                                
Clean Air Act, SNCR [Member] | Public Service Company of New Mexico [Member] | San Juan Generating Station Units 1 and 4 [Member] | Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Total costs for equipment                   $ 76,800,000                    
Clean Air Act, SNCR [Member] | Public Service Company of New Mexico [Member] | San Juan Generating Station Units 2 and 3 [Member]                                        
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             20 years                      
Recovery percentage of estimated undepreciated value at 12/31/17   50.00%   50.00%     50.00%   50.00% 50.00%                    
Estimated undepreciated value at 12/31/17                 $ 255,300,000                      
Regulatory disallowances and restructuring costs                 127,600,000                      
Expense reflected in regulatory disallowances and restructuring costs       $ 400,000 $ 800,000                              
Accumulated plant write-off, disallowance       128,600,000     $ 128,600,000     $ 128,600,000                    
Net carrying value of utility plant       $ 141,600,000     141,600,000     141,600,000                    
Clean Air Act, SNCR [Member] | Public Service Company of New Mexico [Member] | San Juan Generating Station Units 2 and 3 [Member] | Increase in coal mine decommissioning liability [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Regulatory disallowances and restructuring costs                 $ 165,700,000                      
Clean Air Act, SNCR [Member] | Public Service Company of New Mexico [Member] | San Juan Generating Station Unit 4 [Member]                                        
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                                    
Clean Air Act, SNCR [Member] | Public Service Company of New Mexico [Member] | San Juan Generating Station Unit 4 [Member] | Maximum [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Annual cost of Renewable Energy Credits   $ 7,000,000                                    
Clean Air Act, SNCR [Member] | Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station Unit 3 [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Estimated undepreciated value at 12/31/17   $ 152,000,000                                    
Clean Air Act, SNCR [Member] | PNMR and PNM [Member] | Palo Verde Nuclear Generating Station Unit 3 [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Number of megawatts | MW   134                                    
Clean Air Act, SNCR [Member] | PNMR Development [Member] | San Juan Generating Station Unit 4 [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Expense for cost, obligation to reimburse         $ 600,000                              
Potential acquisition of ownership (in megawatts) | MW                           65            
Clean Air Act, SNCR Hearing Examiner Recommended Denial [Member] | Public Service Company of New Mexico [Member] | San Juan Generating Station Unit 4 [Member]                                        
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 [Member] | Public Service Company of New Mexico [Member] | San Juan Generating Station Unit 4 [Member]                                        
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 [Member] | Public Service Company of New Mexico [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Expense reflected in regulatory disallowances and restructuring costs       $ 4,800,000                                
Clean Air Act Related to Post Combustion Controls [Member] | Public Service Company of New Mexico [Member] | Four Corners [Member]                                        
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          
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       $ 90,100,000     $ 90,100,000     $ 90,100,000                    
Clean Air Act Related to Post Combustion Controls [Member] | Public Service Company of New Mexico [Member] | Four Corners Units 4 and 5 (Coal) [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Ownership percentage                             13.00%          
Clean Air Act Related to Post Combustion Controls [Member] | Public Service Company of New Mexico [Member] | Four Corners Units 1, 2 and 3 (Coal) [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Ownership percentage                             0.00%          
Clean Power Plan [Member]                                        
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                
Proposed percentage reduction in CO2 emissions, stayed by the USSC                     32.00%                  
Period of time currently remaining to submit initial plans to meet environmental targets             6 months                          
Period of time currently remaining to submit initial plans to meet environmental targets, including potential extension             2 years 6 months                          
National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] | Public Service Company of New Mexico [Member]                                        
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 [Member] | Public Service Company of New Mexico [Member] | Minimum [Member]                                        
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, 2015 EPA Legal Settlement [Member] [Member] | Public Service Company of New Mexico [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Period of time from state designation, to provide implementation plans 36 months                                      
WEG v OSM Lawsuit [Member] | Public Service Company of New Mexico [Member]                                        
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 [Member] | Public Service Company of New Mexico [Member] | San Juan Generating Station [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Number of claims for relief, filed (in lawsuits) | lawsuit                               2        
Santa Fe generating station [Member] | Public Service Company of New Mexico [Member]                                        
Public Utilities, Commitments And Contingencies [Line Items]                                        
Number of wells with elevated levels of nitrates | well       3     3     3