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Regulation (Narrative) (Details)
$ in Thousands
12 Months Ended 45 Months Ended
Feb. 16, 2016
USD ($)
Jan. 15, 2016
USD ($)
Nov. 05, 2015
USD ($)
Oct. 28, 2015
USD ($)
Aug. 10, 2015
USD ($)
May. 20, 2015
May. 11, 2015
USD ($)
May. 01, 2015
USD ($)
Nov. 14, 2014
USD ($)
Jul. 11, 2014
USD ($)
May. 01, 2014
USD ($)
Dec. 31, 2015
USD ($)
MW
Mar. 31, 2013
Jul. 06, 2016
USD ($)
Jun. 30, 2016
USD ($)
Oct. 19, 2015
USD ($)
Oct. 07, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Over-collection of fuel revenues                       $ 4,023           $ 932    
Property, Plant and Equipment, Net                       2,286,458           1,962,583    
Undepreciated Capital Expenditures                       293,796           414,284    
Future Plant Decommissioning                       81,621           74,577 $ 65,214 $ 62,784
Regulatory assets                       $ 115,127           112,086    
Jointly Owned Utility Plant, Proportionate Ownership Share                       7.00%                
Number of Presidential Permit(s)                       2                
Contract Termination Provision, Period                       2 years                
Minimum                                        
Fixed Fuel Factor Revision Period, in Months                       4 months                
PUCT Docket No. 44633 | Fixed Fuel Factor                                        
Increase (Decrease) in Fixed Fuel Factor, Percentage           24.00%                            
PUCT Docket No. 41852                                        
Reconcilable Fuel Expense                   $ 545,300                    
Fuel Reconciliation Period                         45 months              
Four Corners Generating Station                                        
Future Mine Reclamation Expenses                       $ 19,300                
Anticipated closing date for sale                       Jul. 06, 2016                
Jointly Owned Utility Plant, Proportionate Ownership Share                       7.00%                
PUCT                                        
Period To Relate Back New Rates         155 days                              
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program               $ 1,000     $ 2,000                  
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program     $ 1,000           $ 2,000                      
Fuel Over And Under Materiality Threshold Percentage                       4.00%                
Over-collection of fuel revenues                       $ 100                
PUCT | Montana Power Station                                        
Number of power generation units authorized to be constructed                       4                
Property, Plant and Equipment, Operational Date                       Mar. 20, 2015                
PUCT | Community Solar At Montana Power Station                                        
Electric capacity | MW                       3                
PUCT | Four Corners Generating Station                                        
Purchase and Sale Agreement, Execution Date                       Feb. 17, 2015                
PUCT | Non-Fuel Base Rate                                        
Public Utilities, Requested Rate Increase (Decrease), Amount         $ 71,500                              
PUCT | Maximum                                        
Surcharge Or Refund Period         18 months                              
PUCT | Four Corners Generating Station                                        
Anticipated closing date for sale                       Jul. 06, 2016                
PUCT | Four Corners Generating Station | Scenario, Forecast                                        
Property, Plant and Equipment, Net                             $ 20,000          
Undepreciated Capital Expenditures                             $ 10,000          
Future Plant Decommissioning                           $ 7,000            
Future Mine Reclamation Expenses                           $ 19,300            
NMPRC                                        
Over-collection of fuel revenues                       $ 3,800                
Debt Instrument, Face Amount                                 $ 310,000      
NMPRC | Montana Power Station                                        
Number of power generation units authorized to be constructed                       4                
Property, Plant and Equipment, Operational Date                       Mar. 20, 2015                
NMPRC | Four Corners Generating Station                                        
Purchase and Sale Agreement, Execution Date                       Feb. 17, 2015                
NMPRC | Holloman                                        
Electric capacity | MW                       5                
NMPRC | Non-Fuel Base Rate                                        
Public Utilities, Requested Rate Increase (Decrease), Amount             $ 8,600                          
Public Utilities, Requested Rate Increase (Decrease), Percentage             7.10%                          
Public Utilities, Requested Rate Increase (Decrease), Request Reduction, Amount       $ 6,400                                
NMPRC | Fuel In Base Rate                                        
Public Utilities, Requested Rate Increase (Decrease), Amount             $ (15,400)                          
Public Utilities, Requested Rate Increase (Decrease), Percentage             (21.50%)                          
NMPRC | Four Corners Generating Station                                        
Anticipated closing date for sale                       Jul. 06, 2016                
Jointly Owned Utility Plant, Proportionate Ownership Share                       7.00%                
FERC                                        
Debt Instrument, Face Amount                               $ 310,000        
Line of Credit Facility, Maximum Borrowing Capacity                               400,000        
Debt Issuance Authorization Date Range Start                       Nov. 15, 2015                
Debt Issuance Authorization Date Range End                       Nov. 15, 2017                
FERC | Four Corners Generating Station                                        
Jointly Owned Utility Plant, Entire Ownership of Proportionate Interest, Percentage                       100.00%                
Subsequent Event | PUCT | Non-Fuel Base Rate                                        
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount   $ 63,300                                    
Subsequent Event | NMPRC | Non-Fuel Base Rate                                        
Public Utilities, Recommended Rate Increase (Decrease), Amount $ 640                                      
RGRT | NMPRC                                        
Debt Instrument, Face Amount                                 $ 65,000      
RGRT | FERC                                        
Debt Instrument, Face Amount                               $ 65,000        
Final coal reclamation                                        
Regulatory assets [1],[2]                       $ 9,520           $ 10,702    
Final coal reclamation | PUCT                                        
Regulatory assets                       $ 7,600                
[1] This item is recovered through fuel recovery mechanisms established by tariff.
[2] This item relates to coal reclamation costs associated with Four Corners. See Part II, Item 8, Financial Statements and Supplementary Data, Note C.