Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available-For-Sale Securities
Amortized Cost	$ 54,410	$ 71,485
Gross Unrealized Gain	695	277
Gross Unrealized (Losses)	(82)	(255)
Estimated Fair Value	55,023	71,507
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	26,920	42,584
Gross Unrealized Gain	480	165
Gross Unrealized (Losses)	(8)	(40)
Estimated Fair Value	27,392	42,709
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	21,890	23,301
Gross Unrealized Gain	215	112
Gross Unrealized (Losses)	(74)	(215)
Estimated Fair Value	22,031	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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