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Regulatory and Rate Matters - TNMP Narrative (Details)
advanced_meter in Thousands, $ in Thousands
 1 Months Ended 12 Months Ended
Feb. 06, 2020
USD ($)
Nov. 02, 2018
USD ($)
May 30, 2018
USD ($)
Jan. 10, 2018
USD ($)
Mar. 13, 2017
USD ($)
Dec. 07, 2016
USD ($)
Oct. 31, 2019
USD ($)
Aug. 31, 2019
USD ($)
Jul. 31, 2011
USD ($)
Dec. 31, 2019
USD ($)
advanced_meter
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Public Utilities, General Disclosures [Line Items]                        
Unrecovered investment revenue     $ 20,200                  
Net increase in regulatory liabilities                     $ 7,175 $ 548,952
Public Service Company of New Mexico                        
Public Utilities, General Disclosures [Line Items]                        
Regulatory liabilities                   $ 683,769 719,946  
Net increase in regulatory liabilities                     11,244 402,501
Texas-New Mexico Power Company                        
Public Utilities, General Disclosures [Line Items]                        
Regulatory liabilities                   $ 182,979 180,929  
Net increase in regulatory liabilities                     (4,069) 146,451
Refund of regulatory liability to customers                       37,800
CTC Funding amount interest minimum                   15.00%    
Adjustment for collection of amortization         $ 1,100              
Reduction to revenue                     $ 5,400  
Texas-New Mexico Power Company | 2018 TNMP Rate Case                        
Public Utilities, General Disclosures [Line Items]                        
Requested rate increase (decrease)   $ 10,000 $ 25,900                  
Requested return on equity   9.65% 10.50%                  
Requested cost of debt, percentage   6.44% 7.20%                  
Requested debt capital structure, percentage   55.00% 50.00%                  
Requested equity capital structure, percentage   45.00% 50.00%                  
Regulatory liabilities                       146,500
Net increase in regulatory liabilities                       $ 14,400
Refund of federal income tax rates period                     3 years 5 years
Investments excluded from rate, amount   $ 10,600                    
Recovery of direct costs             $ 3,300 $ 3,800        
Pre-tax write-off                   $ 500    
Texas-New Mexico Power Company | Advanced Meter System Deployment and Surcharge Request                        
Public Utilities, General Disclosures [Line Items]                        
Approved deployment costs                 $ 113,400      
Period of time to collect deployment costs through surcharge period                 12 years      
Number of advanced meters installed | advanced_meter                   242    
Texas-New Mexico Power Company | Transmission Cost of Service Rates | Subsequent Event                        
Public Utilities, General Disclosures [Line Items]                        
Requested rate increase (decrease) $ 59,200                      
Requested increase in annual transmission service revenue $ 7,800                      
NMPRC                        
Public Utilities, General Disclosures [Line Items]                        
Requested rate increase (decrease)           $ 99,200            
Requested return on equity           10.125%            
NMPRC | Public Service Company of New Mexico                        
Public Utilities, General Disclosures [Line Items]                        
Requested rate increase (decrease)       $ 10,300                
Requested return on equity       9.575%