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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
	Mar. 31, 2021	Mar. 31, 2020
Equity securities:
Net gains (losses) from equity securities sold	$ 2,022	$ (1,315)
Net gains (losses) from equity securities still held	(3,166)	(18,931)
Total net gains (losses) on equity securities	(1,144)	(20,246)
Available-for-sale debt securities:
Net gains (losses) on debt securities	2,112	(12,603)
Net gains (losses) on investment securities	968	(32,849)
PNM
Available-for-sale debt securities:
Proceeds from sales	123,596	149,355
Gross realized gains	8,692	5,825
Gross realized (losses)	$ (3,443)	$ (7,035)

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