Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-For-Sale Securities
Amortized Cost	$ 50,372	$ 71,485
Gross Unrealized Gain	760	277
Gross Unrealized (Losses)	(12)	(255)
Estimated Fair Value	51,120	71,507
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	22,865	42,584
Gross Unrealized Gain	485	165
Gross Unrealized (Losses)	0	(40)
Estimated Fair Value	23,350	42,709
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	21,907	23,301
Gross Unrealized Gain	275	112
Gross Unrealized (Losses)	(12)	(215)
Estimated Fair Value	22,170	23,198
Variable rate demand notes
Available-For-Sale Securities
Amortized Cost	5,600	5,600
Gross Unrealized Gain	0	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	$ 5,600	$ 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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