Investments in Available-for-Sale Securities (Cost Basis to Fair Value Reconciliation) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost Basis	$ 135,687	$ 163,140
Gross Unrealized Gains	201	15
Gross Unrealized Losses	(221)	(358)
Fair Value	135,667	162,797
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Cost Basis	35,044	100,003
Gross Unrealized Gains	1	11
Gross Unrealized Losses	(16)	(126)
Fair Value	35,029	99,888
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Cost Basis	100,643	63,137
Gross Unrealized Gains	200	4
Gross Unrealized Losses	(205)	(232)
Fair Value	100,638	62,909
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Fair Value	135,667	162,797
Short-term investments \| U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	35,029	99,888
Short-term investments \| Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 100,638	$ 62,909

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).

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 (b)
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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).

+ References

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-Publisher FASB
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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=118939692&loc=d3e27161-111563

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
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-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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