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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Jan. 01, 2018
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, unrealized loss position $ 0   $ 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 (800,000) $ (2,600,000) 2,600,000 $ (3,800,000)    
Proceeds from sales 159,551,000 167,359,000 234,011,000 794,088,000    
Gross realized gains 10,906,000 7,549,000 15,095,000 13,570,000    
Gross realized (losses) (5,802,000) $ (6,192,000) (8,972,000) $ (10,869,000)    
Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale debt securities 362,793,000   362,793,000   $ 328,242,000  
Nuclear Decommissioning Trust | Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale debt securities 318,800,000   318,800,000   287,100,000  
Mine Reclamation Trust | Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale debt securities $ 44,000,000.0   $ 44,000,000.0   $ 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