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Regulation (Narrative) (Details)
$ in Thousands
3 Months Ended
May 01, 2017
USD ($)
Feb. 22, 2017
USD ($)
Feb. 13, 2017
USD ($)
Jan. 10, 2017
Sep. 27, 2016
USD ($)
Aug. 25, 2016
USD ($)
Jul. 01, 2016
USD ($)
Jun. 08, 2016
USD ($)
Mar. 31, 2017
USD ($)
MW
Dec. 31, 2016
USD ($)
Under-collection of fuel revenues                 $ 2,519 $ 11,123
Over-collection of fuel revenues                 $ 181 $ 255
PUCT                    
Period To Relate Back New Rates                 155 days  
Under-collection of fuel revenues                 $ 2,500  
Reconcilable Fuel Expense                 $ 140,400  
PUCT | Four Corners Generating Station                    
Purchase and Sale Agreement, Execution Date                 Feb. 17, 2015  
PUCT | Community Solar At Montana Power Station                    
Electric Capacity | MW                 3  
Number of customers subscribed                 1,500  
PUCT | Maximum                    
Surcharge Or Refund Period                 18 months  
PUCT | Four Corners Generating Station                    
Closing Date of Sale                 Jul. 06, 2016  
Mine Reclamation Liability, Noncurrent                 $ 7,100  
NMPRC                    
Reconcilable Fuel Expense                 $ 127,500  
Fuel Reconciliation Period                 27 months  
Over-collection of fuel revenues                 $ 200  
NMPRC | Holloman                    
Electric Capacity | MW                 5  
Non-Fuel Base Rate | PUCT                    
Public Utilities, Requested Rate Increase, Amount     $ 42,500              
2016 Energy Efficiency Programs | PUCT | Subsequent Event                    
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program $ 1,000                  
2017 Energy Efficiency Programs | NMPRC                    
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program             $ 400      
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program   $ 400                
2015 Energy Efficiency Programs | NMPRC                    
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program             $ 300      
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program                 $ 300  
Public Utilities, Regulatory Rate Making Impact | PUCT                    
Recovery of rate case expenses           $ 3,100        
Period To Relate Back New Rates           80 days        
Public Utilities, Regulatory Rate Making Impact | NMPRC                    
Public Utilities, Approved Return on Equity, Percentage               9.48%    
Public Utilities, Regulatory Rate Making Impact | Non-Fuel Base Rate | PUCT | Four Corners Generating Station                    
Public Utilities, Approved Rate Increase, Amount           $ 3,700        
Regulatory Current Asset, End Date for Recovery           Jul. 12, 2017        
Public Utilities, Regulatory Rate Making Impact | Non-Fuel Base Rate | NMPRC                    
Public Utilities, Approved Rate Increase, Amount               $ 600    
Public Utilities, Regulatory Rate Making Impact | Utility Cost Of Service | NMPRC                    
Public Utilities, Approved Rate Increase, Amount               $ 500    
PUCT Docket No. 46610 | Fixed Fuel Factor                    
Increase In Fixed Fuel Factor, Percentage       28.80%            
PUCT Docket No. 46308                    
Reconcilable Fuel Expense         $ 436,600          
Fuel Reconciliation Period         36 months          
NMPRC Case No. 15-00127-UT                    
Fuel Reconciliation Period               24 months