Short-Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 56,932	$ 65,715
Gross Unrealized Losses	(3)	(69)
Estimated Fair Value	56,929	65,646
Municipal bonds and notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,986	9,210
Gross Unrealized Losses		(19)
Estimated Fair Value	2,986	9,191
Zero coupon bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33,919	29,823
Gross Unrealized Losses		(28)
Estimated Fair Value	33,919	29,795
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,509	26,175
Gross Unrealized Losses	(3)	(22)
Estimated Fair Value	19,506	26,153
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	518	507
Estimated Fair Value	$ 518	$ 507

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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), classified as current.

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