Short-Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 65,715	$ 141,584
Gross Unrealized Losses	(69)	(122)
Estimated Fair Value	65,646	141,462
Municipal bonds and notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,210	21,414
Gross Unrealized Losses	(19)	(49)
Estimated Fair Value	9,191	21,365
Zero coupon bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,823	54,159
Gross Unrealized Losses	(28)	(33)
Estimated Fair Value	29,795	54,126
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	26,175	55,109
Gross Unrealized Losses	(22)	(40)
Estimated Fair Value	26,153	55,069
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	507	10,902
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 507	$ 10,902

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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-Topic 320
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