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Regulation (Narrative) (Details)
$ in Thousands
3 Months Ended 6 Months Ended
Jul. 21, 2017
USD ($)
Jun. 29, 2017
May 01, 2017
USD ($)
Feb. 22, 2017
USD ($)
Feb. 13, 2017
USD ($)
Jan. 10, 2017
Sep. 27, 2016
USD ($)
Aug. 25, 2016
USD ($)
Jul. 01, 2016
USD ($)
Jun. 08, 2016
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Jun. 30, 2017
USD ($)
MW
Dec. 31, 2016
USD ($)
Under-collection of fuel revenues                     $ 8,515   $ 8,515 $ 11,123
Over-collection of fuel revenues                     314   $ 314 $ 255
PUCT                            
Period To Relate Back New Rates                         155 days  
Under-collection of fuel revenues                     $ 8,500   $ 8,500  
Reconcilable Fuel Expense                         $ 181,400  
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   1,500  
Property, Plant and Equipment, Operational Date                     May 31, 2017      
PUCT | Maximum                            
Surcharge Or Refund Period                         18 months  
PUCT | Four Corners Generating Station                            
Closing Date of Sale                         Jul. 06, 2016  
PUCT | Subsequent Event                            
Public Utilities, Requested Rate Increase (Decrease), Amended Decrease Amount $ 3,000                          
NMPRC                            
Reconcilable Fuel Expense                         $ 144,100  
Fuel Reconciliation Period                         30 months  
Over-collection of fuel revenues                     $ 300   $ 300  
NMPRC | Holloman                            
Electric Capacity | MW                         5  
Non-Fuel Base Rate | PUCT                            
Public Utilities, Requested Rate Increase, Amount         $ 42,500                  
Non-Fuel Base Rate | PUCT | Subsequent Event                            
Public Utilities, Requested Rate Increase, Amended Amount $ 39,200                          
2016 Energy Efficiency Programs | PUCT                            
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program     $ 1,000                      
2016 Energy Efficiency Programs | NMPRC                            
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program                     400      
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. 11, 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         36 months              
Fuel reconciliation effect, net amount                     $ 5,000      
NMPRC Case No. 15-00127-UT                            
Fuel Reconciliation Period                   24 months