Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-For-Sale Securities
Amortized Cost	$ 35,056	$ 37,010
Gross Unrealized Gain	679	817
Gross Unrealized (Losses)	0	0
Estimated Fair Value	35,735	37,827
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	18,917	19,924
Gross Unrealized Gain	324	365
Gross Unrealized (Losses)	0	0
Estimated Fair Value	19,241	20,289
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	16,139	17,086
Gross Unrealized Gain	355	452
Gross Unrealized (Losses)	0	0
Estimated Fair Value	$ 16,494	$ 17,538

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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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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