Investments in Available-for-Sale Securities (Schedule of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Cost Basis	$ 129,544	$ 135,687
Available-for-sale, Gains	245	201
Available-for-sale, Losses	(732)	(221)
Available-for-sale, Fair Value	129,057	135,667
Available-for-sale, Short-Term Investments	129,057	135,667
Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	129,057	135,667
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Cost Basis	38,286	35,044
Available-for-sale, Gains	86	1
Available-for-sale, Losses		(16)
Available-for-sale, Fair Value	38,372	35,029
U.S. government and agency securities \| Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	38,372	35,029
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Cost Basis	91,258	100,643
Available-for-sale, Gains	159	200
Available-for-sale, Losses	(732)	(205)
Available-for-sale, Fair Value	90,685	100,638
Corporate notes and bonds \| Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	$ 90,685	$ 100,638

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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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