Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-For-Sale Securities
Amortized Cost	$ 71,485	$ 75,060
Gross Unrealized Gain	277	57
Gross Unrealized (Losses)	(255)	(318)
Estimated Fair Value - Total	71,507	74,799
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	42,584	48,202
Gross Unrealized Gain	165	12
Gross Unrealized (Losses)	(40)	(284)
Estimated Fair Value - Total	42,709	47,930
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	23,301	22,858
Gross Unrealized Gain	112	45
Gross Unrealized (Losses)	(215)	(34)
Estimated Fair Value - Total	23,198	22,869
Variable rate demand notes
Available-For-Sale Securities
Amortized Cost	5,600	4,000
Estimated Fair Value - Total	$ 5,600	$ 4,000

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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