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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Equity securities:
Net gains from equity securities sold	$ 90	$ 514	$ 4,131	$ 4,675
Net gains from equity securities still held	8,545	1,011	2,991	10,916
Total net gains on equity securities	8,635	1,525	7,122	15,591
Available-for-sale debt securities:
Net gains (losses) on debt securities	5,766	161	(3,950)	4,708
Net gains on investment securities	14,401	1,686	3,172	20,299
PNM
Available-for-sale debt securities:
Proceeds from sales	134,567	141,371	489,218	375,382
Gross realized gains	12,998	6,510	30,085	21,605
Gross realized (losses)	$ (11,374)	$ (6,026)	$ (25,411)	$ (14,998)

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