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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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity securities:
Net gains (losses) from equity securities sold	$ (309)	$ 90	$ 5,931	$ 4,131
Net gains (losses) from equity securities still held	(3,153)	8,545	2,388	2,991
Total net gains (losses) on equity securities	(3,462)	8,635	8,319	7,122
Available-for-sale debt securities:
Net gains (losses) on debt securities	5,410	5,766	7,789	(3,950)
Net gains on investment securities	1,948	14,401	16,108	3,172
PNM
Available-for-sale debt securities:
Proceeds from sales	128,683	134,567	396,870	489,218
Gross realized gains	10,986	12,998	30,290	30,085
Gross realized (losses)	$ (6,527)	$ (11,374)	$ (17,776)	$ (25,411)

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