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Fair Value of Derivative and Other Financial Instruments - Investment in NDT and Gross Realized Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity securities:
Net gains from equity securities sold	$ 5,861	$ 5,698	$ 4,864
Net gains (losses) from equity securities still held	17,707	18,319	(10,523)
Total net gains (losses) on equity securities	23,568	24,017	(5,659)
Available-for-sale debt securities:
Net gains (losses) on debt securities	(1,969)	5,572	(11,517)
Net gains (losses) on investment securities	21,599	29,589	(17,176)
Public Service Company of New Mexico
Available-for-sale debt securities:
Proceeds from sales	590,998	494,528	984,533
Gross realized gains	35,904	25,760	19,358
Gross realized (losses)	$ (28,817)	$ (17,453)	$ (16,624)

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