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Summary of the Business and Significant Accounting Policies - Narrative (Details)
 12 Months Ended
Dec. 31, 2019
USD ($)
Facility
utility
MW
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 29, 2017
Sep. 22, 2017
Sep. 05, 2017
MW
Jun. 30, 2017
USD ($)
Accounting Policies Disclosures [Line Items]              
Number of regulated utilities | utility 2            
Payment defaults $ 0            
Other current assets 44,472,000 $ 54,808,000          
Allowance for equity funds used during construction 9,478,000 10,404,000 $ 9,516,000        
Plant in service $ 7,918,601,000 7,548,581,000          
PNMR              
Accounting Policies Disclosures [Line Items]              
Number of regulated utilities | utility 2            
Public Service Company of New Mexico              
Accounting Policies Disclosures [Line Items]              
Restricted cash deposits             $ 1,000,000.0
Other current assets $ 36,561,000 43,516,000          
Allowance for funds used during construction, capitalized interest $ 5,000,000.0 $ 6,100,000 $ 6,300,000        
Allowance for funds used during construction, capitalized interest rate 2.99% 3.19% 3.14%        
Allowance for equity funds used during construction $ 6,656,000 $ 8,173,000 $ 8,664,000        
Allowance for funds used during construction, capitalized cost of equity rate 3.95% 4.25% 4.30%        
Impairment losses on securities held in the NDT $ 5,700,000 $ 13,700,000 $ 7,100,000        
Plant in service 5,753,267,000 5,623,520,000          
Texas-New Mexico Power Company              
Accounting Policies Disclosures [Line Items]              
Other current assets 1,280,000 1,114,000          
Allowance for funds used during construction, capitalized interest $ 2,400,000 $ 2,300,000 $ 1,200,000        
Allowance for funds used during construction, capitalized interest rate 3.23% 3.32% 3.17%        
Allowance for equity funds used during construction $ 2,800,000 $ 2,200,000 $ 900,000        
Allowance for funds used during construction, capitalized cost of equity rate 3.78% 3.29% 2.29%        
Plant in service $ 1,919,256,000 $ 1,686,119,000          
PNMR Development              
Accounting Policies Disclosures [Line Items]              
Solar distributed generation (in mw) | MW 80         50  
NMRD              
Accounting Policies Disclosures [Line Items]              
Solar distributed generation (in mw) | MW 30            
Plant in service $ 12,400,000            
Renewable energy capacity under contract | MW 85.1            
Megawatts supplying energy to data center (in mw) | MW 80            
Megawatts supplying energy to cooperative (in mw) | MW 1.9            
Megawatts supplying energy to power city (in mw) | MW 1.2            
NMRD | PNMR Development              
Accounting Policies Disclosures [Line Items]              
Ownership percentage         50.00%    
Number of solar facilities | Facility 3            
Solar distributed generation (in mw) | MW 10            
Plant in service $ 24,800,000            
Cash contribution percentage 50.00%            
NMRD | AEP OnSite Partners              
Accounting Policies Disclosures [Line Items]              
Ownership percentage       50.00%      
NMRD | PNMR Development and AEP OnSite              
Accounting Policies Disclosures [Line Items]              
Contribution to construction activities $ 38,300,000 $ 9,000,000.0 $ 4,100,000        
NMRD | Central New Mexico Electric Cooperative              
Accounting Policies Disclosures [Line Items]              
Megawatts supplying energy to cooperative (in mw) | MW 2.0