Investments in Available-for-Sale Securities (Schedule of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Cost Basis	$ 162,349	$ 163,140
Available-for-sale, Gains	341	15
Available-for-sale, Losses	(34)	(358)
Available-for-sale, Fair Value	162,656	162,797
Available-for-sale, Short-Term Investments	162,656	162,797
Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	162,656	162,797
Other Assets
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Other Assets	0	0
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Cost Basis	58,283	100,003
Available-for-sale, Gains	94	11
Available-for-sale, Losses	0	(126)
Available-for-sale, Fair Value	58,377	99,888
U.S. government and agency securities \| Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	58,377	99,888
U.S. government and agency securities \| Other Assets
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Other Assets	0	0
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Cost Basis	104,066	63,137
Available-for-sale, Gains	247	4
Available-for-sale, Losses	(34)	(232)
Available-for-sale, Fair Value	104,279	62,909
Corporate notes and bonds \| Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	104,279	62,909
Corporate notes and bonds \| Other Assets
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Other Assets	$ 0	$ 0

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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-Publisher FASB
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-SubTopic 10
-Section 50
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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).

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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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