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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($)
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
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 (12,700,000) $ 3,400,000  
Proceeds from sales 149,355,000 74,460,000  
Gross realized gains 5,825,000 4,189,000  
Gross realized (losses) (7,035,000) $ (3,170,000)  
Recurring | PNM      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale debt securities 353,268,000   $ 388,832,000
Nuclear Decommissioning Trust | Recurring | PNM      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale debt securities 299,900,000   336,000,000.0
Mine Reclamation Trust | Recurring | PNM      
Debt Securities, Available-for-sale [Line Items]      
Available-for-sale debt securities $ 53,400,000   $ 52,800,000