Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available-For-Sale Securities
Amortized Cost	$ 64,724	$ 75,060
Gross Unrealized Gain	320	57
Gross Unrealized (Losses)	(19)	(318)
Estimated Fair Value	65,025	74,799
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	43,852	48,202
Gross Unrealized Gain	191	12
Gross Unrealized (Losses)	(17)	(284)
Estimated Fair Value	44,026	47,930
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	15,272	22,858
Gross Unrealized Gain	129	45
Gross Unrealized (Losses)	(2)	(34)
Estimated Fair Value	15,399	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
-Name Accounting Standards Codification
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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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-Publisher FASB
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