Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available-For-Sale Securities
Amortized Cost	$ 59,684	$ 75,060
Gross Unrealized Gain	338	57
Gross Unrealized (Losses)	(35)	(318)
Estimated Fair Value	59,987	74,799
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	40,148	48,202
Gross Unrealized Gain	207	12
Gross Unrealized (Losses)	(6)	(284)
Estimated Fair Value	40,349	47,930
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	13,936	22,858
Gross Unrealized Gain	131	45
Gross Unrealized (Losses)	(29)	(34)
Estimated Fair Value	14,038	22,869
Variable rate demand notes
Available-For-Sale Securities
Amortized Cost	5,600	4,000
Gross Unrealized Gain	0	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	$ 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
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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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