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Regulatory and Rate Matters - New Mexico General Rate Cases (Details)
$ in Thousands
 9 Months Ended 12 Months Ended 30 Months Ended
Jan. 31, 2018
USD ($)
Jan. 16, 2018
USD ($)
Jan. 10, 2018
USD ($)
Dec. 31, 2017
USD ($)
May 23, 2017
USD ($)
Dec. 07, 2016
USD ($)
Sep. 30, 2016
USD ($)
Sep. 28, 2016
USD ($)
MW
Aug. 04, 2016
USD ($)
MW
Jan. 15, 2016
USD ($)
lease
MW
Aug. 27, 2015
USD ($)
Jan. 15, 2015
Sep. 30, 2016
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2018
USD ($)
Oct. 31, 2017
USD ($)
May 12, 2017
signatory
Jul. 01, 2016
USD ($)
May 31, 2016
MW
Public Utilities, General Disclosures [Line Items]                                          
Deferred federal income tax                           $ 7,716 $ 119,182 $ 60,892          
Pre-tax regulatory disallowance for capital costs                           36,800     $ 58,000        
Regulatory disallowances and restructuring costs                           65,598 27,036 15,011          
Increase (decrease) in non-fuel revenue requirement $ 10,300                                        
Percent of non-fuel revenue requirement change implemented 50.00%                                        
NMPRC                                          
Public Utilities, General Disclosures [Line Items]                                          
Requested rate increase (decrease)           $ 99,200                              
Requested return on equity           10.125%                              
Number of additional signatories | signatory                                     13    
Proposed term for providing benefits to customers related to reduction in state corporate tax         21 years                                
Pre-tax regulatory disallowance       $ 27,900                     27,900            
Public Service Company of New Mexico                                          
Public Utilities, General Disclosures [Line Items]                                          
Deferred federal income tax                           5,661 73,308 49,123          
Regulatory disallowances and restructuring costs                           66,339 $ 27,036 $ 15,011          
Public Service Company of New Mexico | NMPRC                                          
Public Utilities, General Disclosures [Line Items]                                          
Requested rate increase (decrease)   $ (4,400) $ (9,100)   $ 62,300                                
Requested return on equity         9.575%                                
Initial rate increase (decrease) amount         $ 32,300                                
Proposed term for providing benefits to customers related to reduction in state corporate tax         3 years                                
Requested rate increase (decrease), duplicative amount   $ 4,700                                      
Public Service Company of New Mexico | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4                                          
Public Utilities, General Disclosures [Line Items]                                          
Net book value                           50,000     50,000        
Public Service Company of New Mexico | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4 | Installation costs including construction management, gross receipts taxes, AFUDC, and other PNM costs                                          
Public Utilities, General Disclosures [Line Items]                                          
Portion of costs for SNCRs and BDT equipment                 $ 52,300                        
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station, Unit 2 Leases                                          
Public Utilities, General Disclosures [Line Items]                                          
Number of megawatts purchased (in megawatts) | MW                   64.1                     64.1
Number of leases under which assets were purchased | lease                   3                      
Estimated annual property tax expense                 $ 800                        
Number of leases under which lease term was extended | lease                   1                      
Lease term extension period                   8 years                      
Number of megawatts | MW               114.6 114.6                        
Net book value                           73,300     73,300        
Net book value of capital improvements                           38,000     38,000        
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended                                          
Public Utilities, General Disclosures [Line Items]                                          
Estimated annual property tax expense                   $ 1,500                      
Estimated annual rent expense                 $ 18,100                        
Number of leases under which lease term was extended | lease                   4                      
Lease term extension period                       8 years                  
Public Service Company of New Mexico | 2014 Electric Rate Case                                          
Public Utilities, General Disclosures [Line Items]                                          
Future test year period           12 months                              
Redefined future test year as period that begins a period of time following the filing of a rate case application (up to)           13 months                              
Public Service Company of New Mexico | 2015 Electric Rate Case                                          
Public Utilities, General Disclosures [Line Items]                                          
Requested rate increase (decrease)                     $ 123,500                    
Requested return on equity                     10.50%                    
Requested rate increase (decrease) in fuel-related costs                         $ 42,900                
Change in requested rate increase (decrease) in non-fuel related revenues                           200              
Requested rate increase (decrease) in non-fuel related revenue                 121,500         121,500              
Proposed increase (decrease) in non-fuel revenue                 $ 41,300                        
Proposed return on equity                 9.575%                        
Approved rate increase (decrease)               $ 61,200                          
Estimated period of time for New Mexico Supreme Court Appeal decision             15 months               7 months            
Minimum amount of loss of capital cost recovery             15 months                            
Estimate of possible loss                           146,100     $ 146,100        
Public Service Company of New Mexico | 2015 Electric Rate Case | San Juan Generating Station | Refined Coal                                          
Public Utilities, General Disclosures [Line Items]                                          
Proposed credit to retail customers                 100.00%                        
Deferred federal income tax                 $ 2,100                        
Public Service Company of New Mexico | 2015 Electric Rate Case | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4                                          
Public Utilities, General Disclosures [Line Items]                                          
Amount of equipment costs included in rate base                 40,000                        
Public Service Company of New Mexico | 2015 Electric Rate Case | Palo Verde Nuclear Generating Station, Unit 2 Leases                                          
Public Utilities, General Disclosures [Line Items]                                          
Proposed disallowance of recovery of purchase price of assets                 163,300                        
Approved lease acquisition costs               83,700                          
Disallowed leasehold improvements               43,800                          
Pre-tax regulatory disallowance for capital costs       $ 3,100     $ 6,800             4,000              
Public Service Company of New Mexico | 2015 Electric Rate Case | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended                                          
Public Utilities, General Disclosures [Line Items]                                          
Approved recovery of assumed operating and maintenance expense savings annually related to BDT               $ 300                          
Public Service Company of New Mexico | 2015 Electric Rate Case | Alvarado square                                          
Public Utilities, General Disclosures [Line Items]                                          
Proposed disallowance of recovery of assets and deferred charges from retail customers                 $ 4,500                        
Pre-tax regulatory disallowance of costs recorded as regulatory assets and deferred charges                           $ 4,500              
Public Service Company of New Mexico | 2015 Electric Rate Case | Non-Fuel Energy                                          
Public Utilities, General Disclosures [Line Items]                                          
Requested rate increase (decrease)                     $ 121,700                    
Four Corners                                          
Public Utilities, General Disclosures [Line Items]                                          
Pre-tax regulatory disallowance                                   $ 90,100   $ 148,100  
Regulatory disallowances and restructuring costs     $ 47,600