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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 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 200,000 $ (800,000) 2,800,000 $ (4,600,000)    
Proceeds from sales 141,371,000 117,801,000 375,382,000 911,899,000    
Gross realized gains 6,510,000 3,460,000 21,605,000 17,030,000    
Gross realized (losses) (6,026,000) $ (3,149,000) (14,998,000) $ (14,018,000)    
Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale debt securities 369,017,000   369,017,000   $ 328,242,000  
Nuclear Decommissioning Trust | Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale debt securities 322,200,000   322,200,000   287,100,000  
Mine Reclamation Trust | Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale debt securities $ 46,800,000   $ 46,800,000   $ 41,100,000  
Retained Earnings            
Debt Securities, Available-for-sale [Line Items]            
Cumulative effect adjustment           $ 11,208,000
Retained Earnings | PNM            
Debt Securities, Available-for-sale [Line Items]            
Cumulative effect adjustment           11,208,000
Accounting Standards Update 2016-01 | Retained Earnings            
Debt Securities, Available-for-sale [Line Items]            
Cumulative effect adjustment           $ 11,200,000