Fair Value of Derivative and Other Financial Instruments - Investments in NDT (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Equity securities:
Net gains (losses) from equity securities sold	$ 5,356	$ 2,774	$ 4,041	$ 4,161
Net gains (losses) from equity securities still held	13,377	303	(5,554)	9,905
Total net gains (losses) on equity securities	18,733	3,077	(1,513)	14,066
Available-for-sale debt securities:
Net gains (losses) on debt securities	2,887	1,522	(9,716)	4,547
Net gains (losses) on investment securities	$ 21,620	$ 4,599	$ (11,229)	$ 18,613

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Amount of unrealized and realized gain (loss) on investment in debt and equity securities.

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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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