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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Jan. 01, 2018
Debt Securities, Available-for-sale [Line Items]        
Debt securities, available-for-sale, unrealized loss position $ 0      
Other than temporary impairments 0      
PNM        
Debt Securities, Available-for-sale [Line Items]        
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings 3,400,000 $ (1,200,000)    
Proceeds from sales 74,460,000 626,729,000    
Gross realized gains 4,189,000 6,021,000    
Gross realized (losses) (3,170,000) $ (4,677,000)    
Recurring | PNM        
Debt Securities, Available-for-sale [Line Items]        
Available-for-sale debt securities 351,563,000   $ 328,242,000  
Nuclear Decommissioning Trust | Recurring | PNM        
Debt Securities, Available-for-sale [Line Items]        
Available-for-sale debt securities 308,000,000   287,100,000  
Mine Reclamation Trust | Recurring | PNM        
Debt Securities, Available-for-sale [Line Items]        
Available-for-sale debt securities $ 43,600,000   $ 41,100,000  
Retained Earnings        
Debt Securities, Available-for-sale [Line Items]        
Cumulative effect adjustment (Note 7)       $ 11,208,000
Retained Earnings | PNM        
Debt Securities, Available-for-sale [Line Items]        
Cumulative effect adjustment (Note 7)       11,208,000
Accounting Standards Update 2016-01 | Retained Earnings        
Debt Securities, Available-for-sale [Line Items]        
Cumulative effect adjustment (Note 7)       $ 11,200,000