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Significant Accounting Policies and Responsibility for Financial Statements (Details)
 1 Months Ended 3 Months Ended
Feb. 28, 2018
USD ($)
Mar. 31, 2018
USD ($)
MW
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Sep. 22, 2017
Jun. 29, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jul. 08, 2016
MW
Dec. 31, 2015
USD ($)
Business Acquisition [Line Items]                  
Payment defaults under agreements   $ 0              
Dividends declared on common stock   21,108,000              
Revenues   317,878,000 $ 330,178,000            
Total operating expenses   271,746,000 272,080,000            
Net earnings   14,990,000 22,862,000            
Accounts Receivable, Net, Current   84,156,000   $ 90,473,000          
Construction work in progress   284,870,000   245,933,000          
Owners' equity   1,683,614,000   1,695,253,000          
Cash and cash equivalents   $ 3,783,000 3,283,000 3,974,000     $ 5,522,000    
NMRD                  
Business Acquisition [Line Items]                  
Renewable energy capacity in operating (in mw) | MW   21.8              
Construction of solar generation capacity (in megawatts) | MW   10              
PNM                  
Business Acquisition [Line Items]                  
Revenues   $ 236,232,000 251,558,000            
Total operating expenses   207,940,000 211,089,000            
Net earnings   7,837,000 16,658,000            
Accounts Receivable, Net, Current   61,908,000   67,227,000          
Construction work in progress   214,139,000   204,079,000          
Owners' equity   1,430,161,000   1,422,174,000          
Third party deposits           $ 1,000,000 7,200,000   $ 8,200,000
Cash and cash equivalents   19,000 1,094,000 1,108,000     1,324,000    
Texas-New Mexico Power Company                  
Business Acquisition [Line Items]                  
Dividends declared on common stock   1,024,000 9,900,000            
Revenues   81,646,000 78,620,000            
Total operating expenses   63,114,000 60,655,000            
Net earnings   9,413,000 7,604,000            
Accounts Receivable, Net, Current   22,248,000   23,246,000          
Construction work in progress   61,695,000   34,350,000          
Owners' equity   $ 642,794,000   634,405,000          
PNMR Development                  
Business Acquisition [Line Items]                  
Construction of solar generation capacity (in megawatts) | MW   10           10  
PNMR Development | NMRD                  
Business Acquisition [Line Items]                  
Ownership percentage         50.00%        
NMRD                  
Business Acquisition [Line Items]                  
Revenues   $ 400,000              
Net earnings   100,000              
Cash   3,900,000              
Accounts Receivable, Net, Current   400,000              
Construction work in progress       43,300,000          
Accounts payable       4,500,000          
Owners' equity       $ 43,100,000          
PNMR Development and AEP OnSite | NMRD                  
Business Acquisition [Line Items]                  
Contribution to construction activities $ 5,000,000                
Accounting Standards Update 2016-18 | PNM                  
Business Acquisition [Line Items]                  
Cash and cash equivalents     1,000,000       1,000,000    
Accounting Standards Update 2016-18 | NMCUC                  
Business Acquisition [Line Items]                  
Cash and cash equivalents   $ 1,500,000 $ 100,000       $ 1,400,000