Fair Value of Derivative and Other Financial Instruments - Investment in NDT and Gross Realized Losses (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Equity securities:
Net gains (losses) from equity securities sold	$ 1,086	$ (6,940)	$ 8,738
Net gains (losses) from equity securities still held	14,152	(38,025)	(442)
Total net gains (losses) on equity securities	15,238	(44,965)	8,296
Available-for-sale debt securities:
Net gains (losses) on debt securities	4,008	(33,392)	8,554
Net gains (losses) on investment securities	19,246	(78,357)	16,850
PNM
Available-for-sale debt securities:
Net gains (losses) on debt securities	(19,100)	25,800	(700)
Proceeds from sales	574,199	526,448	459,867
Gross realized gains	18,618	22,071	39,408
Gross realized (losses)	$ (32,649)	$ (36,623)	$ (22,815)

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