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Commitments and Contingencies - The Clean Air Act (Details)
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Feb. 25, 2016
Dec. 31, 2015
USD ($)
Dec. 16, 2015
USD ($)
MWh
MW
Jul. 31, 2015
USD ($)
MW
Mar. 02, 2015
lb / MMBTU
T
Dec. 31, 2015
USD ($)
Jun. 30, 2017
USD ($)
lbsofnox / mmbtu
joint_owner
requirement
MW
Jun. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Jun. 30, 2017
USD ($)
lbsofnox / mmbtu
joint_owner
requirement
MW
Dec. 31, 2016
USD ($)
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Jan. 26, 2016
state
Oct. 01, 2015
parts_per_billion
Sep. 30, 2015
parts_per_billion
May 14, 2015
lb / MMBTU
Dec. 30, 2013
Apr. 30, 2013
well
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 $ 0   $ 0   $ 774,000                        
Plant in service, held for future use, and to be abandoned             7,081,606,000     7,081,606,000 $ 6,944,534,000   $ 6,944,534,000                      
Accumulated depreciation and amortization             2,395,590,000     2,395,590,000 2,334,938,000   2,334,938,000                      
PNM                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Regulatory disallowances and restructuring costs             0 $ 0   0   $ 774,000                        
Plant in service, held for future use, and to be abandoned             5,420,475,000     5,420,475,000 5,359,211,000   5,359,211,000                      
Accumulated depreciation and amortization             $ 1,854,466,000     $ 1,854,466,000 1,809,528,000   1,809,528,000                      
PNM | Reeves Station                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Requirements to be implemented | requirement             0     0                            
PNM | 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             209,000,000     209,000,000                            
Net book value             $ 262,800,000     $ 262,800,000                            
PNM | Palo Verde Nuclear Generating Station Unit 3                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Number of megawatts | MW             134     134                            
PNM | 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 (up to)             $ 101,400,000     $ 101,400,000                            
PNM | 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                
PNM | 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 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 | PNM                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Estimated costs to remain unrecovered     $ 20,000,000                                          
Clean Air Act, SNCR | PNM | San Juan Generating Station Units 1 and 4 | Plan                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Ownership percentage       66.30%                                        
Clean Air Act, SNCR | PNM | 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 | PNM | 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%     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             $ 134,200,000     $ 134,200,000                            
Clean Air Act, SNCR | PNM | 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 | PNM | 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 | PNM | 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 | PNM | San Juan Generating Station Unit 4 | Plan                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Ownership percentage       77.30%                                        
Clean Air Act, SNCR | PNM | Palo Verde Nuclear Generating Station Unit 3                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Number of megawatts | MW     134                                          
Estimated undepreciated value at 12/31/17     $ 155,000,000                                          
Clean Air Act, SNCR | PNM | 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 Development | San Juan Generating Station Unit 4                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Additional ownership to be obtained (in megawatts) | MW       65                                        
Expense reflected in regulatory disallowances and restructuring costs                 $ 600,000                              
Potential acquisition of ownership (in megawatts) | MW       65                                        
Clean Air Act, SNCR Hearing Examiner, Recommended Denial | PNM | 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 | PNM | 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 | PNM                                                
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 | PNM | 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 (up to)             $ 90,300,000     $ 90,300,000                            
Clean Air Act related to post combustion controls | PNM | 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 | PNM | 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 | PNM                                                
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 | PNM | 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 | PNM | 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 | PNM                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Period of time from state designation, to provide implementation plans 36 months                                              
WEG v OSM lawsuit | PNM                                                
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 | PNM | 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 | PNM                                                
Public Utilities, Commitments And Contingencies [Line Items]                                                
Number of wells with elevated levels of nitrates | well                                     3