Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-For-Sale Securities
Amortized Cost	$ 37,010	$ 71,485
Gross Unrealized Gain	817	277
Gross Unrealized (Losses)	0	(255)
Estimated Fair Value	37,827	71,507
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	17,086	42,584
Gross Unrealized Gain	452	165
Gross Unrealized (Losses)	0	(40)
Estimated Fair Value	17,538	42,709
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	19,924	23,301
Gross Unrealized Gain	365	112
Gross Unrealized (Losses)	0	(215)
Estimated Fair Value	$ 20,289	23,198
Variable rate demand notes
Available-For-Sale Securities
Amortized Cost		5,600
Gross Unrealized Gain		0
Gross Unrealized (Losses)		0
Estimated Fair Value		$ 5,600

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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