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Commitments and Contingencies - Coal Supply (Details)
 3 Months Ended 12 Months Ended
Feb. 01, 2016
USD ($)
Jan. 31, 2016
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Apr. 20, 2018
USD ($)
Feb. 01, 2018
Oct. 20, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Jun. 29, 2017
USD ($)
Feb. 01, 2017
Dec. 31, 2016
USD ($)
Aug. 01, 2016
payment
Dec. 31, 2015
USD ($)
Public Utilities, Commitments And Contingencies [Line Items]                            
Other current assets     $ 46,389,000 $ 47,358,000                    
PNM                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Other current assets     41,607,000 39,904,000                    
Amount held in a restricted bank account                   $ 1,000,000   $ 7,200,000   $ 8,200,000
PNM | Surface                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Regulatory assets     100,000,000                      
PNM | Four Corners                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Share of estimated shortfall payment               $ 11,600,000 $ 6,500,000          
PNM | Loss on long-term purchase commitment | Surface                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Loss contingency accrual     41,200,000 41,400,000                    
Increase (decrease) for interim storage costs liability       35,000,000                    
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     15,100,000 14,700,000                    
Increase (decrease) for interim storage costs liability       5,000,000                    
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Estimate of possible loss     99,700,000                      
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Estimate of possible loss     127,100,000                      
Coal supply | PNM | San Juan Generating Station                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Other current assets     26,300,000 26,300,000                    
Estimated increase in coal cost                 51.00%          
Coal supply | NM Capital | San Juan Generating Station                            
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%       12.25%         9.25%      
Requirement to post reclamation bonds     118,700,000                      
Cash used to support bank letter or credit arrangement   $ 30,300,000 30,300,000                      
Coal supply | NM Capital | San Juan Coal Company, Westmoreland                            
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 51,000,000                      
Scheduled principal payment proceeds     900,000                      
Scheduled interest payment proceeds     1,800,000                      
Prepayment penalty     0                      
Coal supply | Unaffiliated third party | San Juan Coal Company, Westmoreland                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Amount held in a restricted bank account             $ 2,700,000              
BTMU Term Loan Agreement | NM Capital                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Long-term Debt $ 125,000,000.0   45,100,000                      
Subsequent event | Coal supply | Unaffiliated third party | San Juan Coal Company, Westmoreland                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Amount held in a restricted bank account         $ 2,700,000                  
Mine Reclamation Trust | PNM                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Required contribution to Reclamation Trust, current fiscal year     7,500,000                      
Reclamation Trust Funding, Year 2     8,700,000                      
Reclamation Trust Funding, Year 3     9,200,000                      
San Juan Generating Station | Loss on long-term purchase commitment | PNM                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Annual funding post-term reclamation trust       $ 5,800,000                    
Four Corners | Mine Reclamation Trust | PNM                            
Public Utilities, Commitments And Contingencies [Line Items]                            
Required contribution to Reclamation Trust, current fiscal year     2,300,000                      
Reclamation Trust Funding, Year 2     2,300,000                      
Reclamation Trust Funding, Year 3     $ 2,100,000                      
Number of annual installment payments | payment                         13