Financial Instruments - Summary of Investments (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available for Sale [Roll Forward]
Fair value at beginning of period	$ 25,708	$ 60,659
Purchases	13,050	3,447
Redemptions	(6,280)	(22,078)
Maturities	(4,520)	(15,409)
Change in bond premium	874	(857)
Fair market value adjustment	(377)	(54)
Fair value at end of period	$ 28,455	$ 25,708

Available-for-sale securities, debt securities, fair market value adjustment.

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Debt Securities, Available for Sale

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Increase (Decrease) in bond premium.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of cash outflow to acquire investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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