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Commitments and Contingencies - Coal Supply (Details)
 3 Months Ended 6 Months Ended 12 Months Ended
Feb. 01, 2016
USD ($)
Jan. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Jul. 25, 2017
USD ($)
Feb. 01, 2017
Aug. 01, 2016
payment
Public Utilities, Commitments And Contingencies [Line Items]                        
Other current assets       $ 65,400,000   $ 65,400,000 $ 73,444,000   $ 73,444,000      
Regulatory disallowances and restructuring costs       0 $ 0 0   $ 774,000        
PNM                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Other current assets       59,357,000   59,357,000 67,355,000   67,355,000      
Regulatory disallowances and restructuring costs       0 $ 0 0   $ 774,000        
PNM | Increase in coal mine decommissioning liability                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Expense reflected in regulatory disallowances and restructuring costs             4,500,000   4,500,000      
PNM | Mine Reclamation Trust                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Required contribution to Reclamation Trust, current fiscal year       6,300,000   6,300,000            
Anticipated funding to Reclamation Trust, next fiscal year       8,300,000   8,300,000            
Anticipated funding to Reclamation Trust, third fiscal year       8,700,000   8,700,000            
PNM | Mine Reclamation Trust | Four Corners                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Required contribution to Reclamation Trust, current fiscal year       2,000,000   2,000,000            
Anticipated funding to Reclamation Trust, next fiscal year       2,100,000   2,100,000            
Anticipated funding to Reclamation Trust, third fiscal year       2,100,000   2,100,000            
Number of annual installment payments | payment                       13
PNM | Surface                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Regulatory assets       100,000,000   100,000,000            
PNM | Loss on long-term purchase commitment | Surface                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Loss contingency accrual       41,400,000   41,400,000 41,000,000   41,000,000      
Final reclamation, capped amount to be collected           100,000,000.0            
PNM | Loss on long-term purchase commitment | Underground                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Loss contingency accrual       14,300,000   14,300,000 14,000,000   14,000,000      
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       101,400,000   101,400,000            
PNM | San Juan Generating Station | Loss on long-term purchase commitment | Underground                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Estimate of possible loss       127,400,000   127,400,000            
PNM | San Juan Generating Station | Coal supply                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Other current assets       $ 37,400,000   $ 37,400,000 $ 48,700,000   $ 48,700,000      
Estimated increase in coal cost       51.00%   51.00%            
PNM | Four Corners                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Share of estimated shortfall payment       $ 6,500,000   $ 6,500,000            
NM Capital | BTMU Term Loan Agreement                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Term loans $ 125,000,000.0     71,800,000   71,800,000            
NM Capital | San Juan Generating Station | Coal supply                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Payments to fund long-term loans to unaffiliated third party   $ 125,000,000.0                    
Loan agreement among several entities   $ 125,000,000.0                    
Interest rate   7.25%                 9.25%  
Requirement to post reclamation bonds       118,700,000   118,700,000            
Cash used to support bank letter or credit arrangement   $ 30,300,000   30,300,000   30,300,000            
NM Capital | San Juan Coal Company, Westmoreland | Coal supply                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Payments to fund long-term loans to unaffiliated third party $ 125,000,000.0                      
Loan receivable   $ 125,000,000   75,800,000   75,800,000            
Scheduled principal payment proceeds       9,600,000   9,600,000            
Scheduled interest payment proceeds       2,000,000   2,000,000            
Prepayment penalty       $ 0   $ 0            
Unaffiliated third party | San Juan Coal Company, Westmoreland | Coal supply | Subsequent event                        
Public Utilities, Commitments And Contingencies [Line Items]                        
Amount held in a restricted bank account                   $ 11,600,000