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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, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
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 4,000,000.0 $ (800,000) (8,700,000) $ 2,600,000  
Proceeds from sales 205,296,000 159,551,000 354,651,000 234,011,000  
Gross realized gains 11,262,000 10,906,000 17,087,000 15,095,000  
Gross realized (losses) (7,002,000) $ (5,802,000) (14,037,000) $ (8,972,000)  
Recurring | PNM          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale debt securities 389,452,000   389,452,000   $ 388,832,000
Nuclear Decommissioning Trust | Recurring | PNM          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale debt securities 335,500,000   335,500,000   336,000,000.0
Mine Reclamation Trust | Recurring | PNM          
Debt Securities, Available-for-sale [Line Items]          
Available-for-sale debt securities $ 53,900,000   $ 53,900,000   $ 52,800,000