Marketable Investments - Summary of Company's Marketable Investments (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 54,521
Gross Unrealized Losses	(188)
Market Value	54,333
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	52,721
Gross Unrealized Losses	(181)
Market Value	52,540
Federal Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,800
Gross Unrealized Losses	(7)
Market Value	$ 1,793

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 2
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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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