Short-Term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 16,527	$ 56,932
Gross Unrealized Gains	2
Gross Unrealized Losses		(3)
Estimated Fair Value	16,529	56,929
Municipal bonds and notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		2,986
Gross Unrealized Losses		0
Estimated Fair Value		2,986
Zero coupon bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,995	33,919
Gross Unrealized Gains	2
Gross Unrealized Losses		0
Estimated Fair Value	13,997	33,919
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,002	19,509
Gross Unrealized Gains	0
Gross Unrealized Losses		(3)
Estimated Fair Value	2,002	19,506
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	530	518
Gross Unrealized Gains	0
Gross Unrealized Losses		0
Estimated Fair Value	$ 530	$ 518

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

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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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