Marketable Investments - Summary of Company's Marketable Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 54,521
Gross Unrealized Losses		(188)
Market Value	$ 52,395	54,333
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	52,567	52,721
Gross Unrealized Gains	1
Gross Unrealized Losses	(173)	(181)
Market Value	$ 52,395	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 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).

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-Publisher FASB
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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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