Marketable Securities - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-For-Sale Securities
Amortized Cost	$ 68,272	$ 75,060
Gross Unrealized Gain	178	57
Gross Unrealized (Losses)	(98)	(318)
Estimated Fair Value	68,352	74,799
Corporate debt securities
Available-For-Sale Securities
Amortized Cost	46,685	48,202
Gross Unrealized Gain	95	12
Gross Unrealized (Losses)	(89)	(284)
Estimated Fair Value	46,691	47,930
Tax-exempt municipal securities
Available-For-Sale Securities
Amortized Cost	18,969	22,858
Gross Unrealized Gain	83	45
Gross Unrealized (Losses)	(9)	(34)
Estimated Fair Value	19,043	22,869
Variable rate demand notes
Available-For-Sale Securities
Amortized Cost	1,600	4,000
Gross Unrealized Gain	0	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	1,600	$ 4,000
Commercial paper
Available-For-Sale Securities
Amortized Cost	1,018
Gross Unrealized Gain	0
Gross Unrealized (Losses)	0
Estimated Fair Value	$ 1,018

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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-Section 50
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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
-Name Accounting Standards Codification
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