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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 | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Equity securities: |  |  |  |
Net gains from equity securities sold | $ 4,218 | $ 5,356 | $ 6,240 | $ 4,041
Net gains (losses) from equity securities still held | 8,707 | 13,377 | 5,541 | (5,554)
Total net gains (losses) on equity securities | 12,925 | 18,733 | 11,781 | (1,513)
Available-for-sale debt securities: |  |  |  |
Net gains (losses) on debt securities | 267 | 2,887 | 2,379 | (9,716)
Net gains (losses) on investment securities | 13,192 | 21,620 | 14,160 | (11,229)
PNM |  |  |  |
Available-for-sale debt securities: |  |  |  |
Proceeds from sales | 239,696 | 205,296 | 363,291 | 354,651
Gross realized gains | 10,611 | 11,262 | 19,304 | 17,087
Gross realized (losses) | $ (7,805) | $ (7,002) | $ (11,249) | $ (14,037)

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