Investments in Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Cost Basis	$ 161,374	$ 156,189
Available-for-sale, Gains	6	22
Available-for-sale, Losses	(486)	(365)
Available-for-sale, Fair Value	160,894	155,846
Available-for-sale, Short-Term Investments	160,894	155,846
Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	160,894	155,846
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	88,858	64,151
Available-for-sale, Gains	3	22
Available-for-sale, Losses	(161)	(73)
Available-for-sale, Fair Value	88,700	64,100
U.S. government and agency securities \| Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	88,700	64,100
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	72,516	92,038
Available-for-sale, Gains	3	0
Available-for-sale, Losses	(325)	(292)
Available-for-sale, Fair Value	72,194	91,746
Corporate notes and bonds \| Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Short-Term Investments	72,194	91,746
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
-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).

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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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