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Commitments and Contingencies - The Clean Air Act (Details)
$ in Thousands
Dec. 16, 2015
MW
Mar. 02, 2015
lb / MMBTU
T
Mar. 31, 2018
USD ($)
lbsofnox / mmbtu
joint_owner
MW
Dec. 31, 2017
USD ($)
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
Jul. 31, 2005
T
Dec. 31, 1999
state
Public Utilities, Commitments And Contingencies [Line Items]                            
Plant in service and held for future use     $ 7,284,727 $ 7,238,285                    
Accumulated depreciation and amortization     $ 2,627,329 2,592,692                    
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 Power Plan                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Number of states that filed a petition against the Clean Power Plan | state         29                  
San Juan Generating Station Unit 4 | Clean Air Act, SNCR                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Number of other joint owners | joint_owner     8                      
PNM                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Plant in service and held for future use     $ 5,537,638 5,501,070                    
Accumulated depreciation and amortization     $ 2,052,380 $ 2,029,534                    
Power to be sold to third party (in megawatts) | MW     36                      
PNM | National Ambient Air Quality Standards, 2015 EPA Legal Settlement                            
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                        
PNM | WEG v OSM lawsuit                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Number of mines impacted | mine                     7      
Number of claims for relief, filed | lawsuit                     15      
Number of states impacted | state                     4      
PNM | Santa Fe generating station                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Number of wells with elevated levels of nitrates | well                   3        
PNM | San Juan Generating Station | WEG v OSM lawsuit                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Number of claims for relief, filed | lawsuit                     2      
PNM | San Juan Generating Station Unit 4 | Clean Air Act, SNCR                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Additional ownership to be obtained (in megawatts) | MW 65                          
PNM | San Juan Generating Station Unit 4 | Clean Air Act, Post-2022 coal supply future rate case                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Period of time agreed to by all parties to settle rate case 6 months                          
PNM | San Juan Generating Station Unit 4 | Clean Air Act, SNCR Hearing Examiner, Recommended Denial                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Additional ownership to be obtained (in megawatts) | MW 132                          
PNM | Palo Verde Nuclear Generating Station Unit 3 | Clean Air Act, SNCR                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Number of megawatts nuclear generation (in megawatts) | MW 134                          
PNM | Four Corners | Clean Air Act related to post combustion controls                            
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     $ 89,000                      
PNM | Four Corners Units 4 and 5 (Coal) | Clean Air Act related to post combustion controls                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Ownership percentage     13.00%                      
PNM | Four Corners Units 1, 2 and 3 (Coal) | Clean Air Act related to post combustion controls                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Ownership percentage                 0.00%          
Maximum | PNM | San Juan Generating Station and Four Corners                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Government standard emission limit (in ozone parts per million) | parts_per_billion           70 75              
Minimum | PNM | National Ambient Air Quality Standards, 2015 EPA Legal Settlement                            
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                        
Surface | Loss on long-term purchase commitment | PNM | San Juan Generating Station                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Estimate of possible loss     $ 99,700                      
San Juan Generating Station | PNM | National Ambient Air Quality Standards                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Revised SO2 emissions (in pounds per MMBTU) | lb / MMBTU               0.10