Fair Value Measurements - Marketable Debt Securities (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Mar. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Amortized Cost		$ 120,543	$ 117,970
Gross Unrealized Gains		5,316	7,043
Gross Unrealized Losses		(276)	(1)
Estimated Fair Value		125,583	125,012
Other-than-temporary Impairments	$ 0	0
Municipal bonds — available-for-sale
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Amortized Cost		120,543	117,970
Gross Unrealized Gains		5,316	7,043
Gross Unrealized Losses		(276)	(1)
Estimated Fair Value		125,583	$ 125,012
Other-than-temporary Impairments	$ 0	$ 0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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-Paragraph 2
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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

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