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Fair Value of Derivative and Other Financial Instruments - Investments in NDT and Gross Realized Losses (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Equity securities: |  |  |  |
Net gains from equity securities sold | $ 90 | $ 514 | $ 4,131 | $ 4,675
Net gains from equity securities still held | 8,545 | 1,011 | 2,991 | 10,916
Total net gains on equity securities | 8,635 | 1,525 | 7,122 | 15,591
Available-for-sale debt securities: |  |  |  |
Net gains (losses) on debt securities | 5,766 | 161 | (3,950) | 4,708
Net gains on investment securities | 14,401 | 1,686 | 3,172 | 20,299
PNM |  |  |  |
Available-for-sale debt securities: |  |  |  |
Proceeds from sales | 134,567 | 141,371 | 489,218 | 375,382
Gross realized gains | 12,998 | 6,510 | 30,085 | 21,605
Gross realized (losses) | $ (11,374) | $ (6,026) | $ (25,411) | $ (14,998)

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