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Commitments and Contingencies - The Clean Air Act (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2019
USD ($)
option
T
Feb. 25, 2019
parts_per_billion
Dec. 31, 2018
USD ($)
MW
Feb. 09, 2016
state
Oct. 01, 2015
parts_per_billion
Sep. 30, 2015
parts_per_billion
May 14, 2015
lb / MMBTU
Apr. 30, 2013
well
Feb. 28, 2013
claim
state
mine
Dec. 31, 1999
state
Public Utilities, Commitments And Contingencies [Line Items]                    
Number of options for meeting BTA standards | option 7                  
Expected number of additional monitoring wells 38                  
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            
PNM | WEG v OSM lawsuit                    
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 | claim                 15  
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 Unit 4 | Clean Air Act, SNCR                    
Public Utilities, Commitments And Contingencies [Line Items]                    
Pre-tax impairment of investments     $ 35.0              
Undepreciated investment in ownership to be obtained     11.9              
Forecasted undepreciated investment     $ 23.1              
Remaining undepreciated investment $ 371.6                  
PNM | San Juan Generating Station | Clean Air Act, SNCR                    
Public Utilities, Commitments And Contingencies [Line Items]                    
Additional ownership to be obtained (in megawatts) | MW     132              
PNM | San Juan Generating Station | WEG v OSM lawsuit                    
Public Utilities, Commitments And Contingencies [Line Items]                    
Number of claims for relief, filed | claim                 2  
PNMR Development | San Juan Generating Station Unit 4 | Clean Air Act, SNCR                    
Public Utilities, Commitments And Contingencies [Line Items]                    
Potential acquisition of ownership (in megawatts) | MW     65              
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   75     70 75        
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