Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 75,641	$ 95,166
Gross Unrealized Gain	18	2
Gross Unrealized Loss	(88)	(283)
Fair Value	75,571	94,885
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,145	6,167
Fair Value	4,145	6,167
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,488	15,971
Gross Unrealized Loss	(11)	(23)
Fair Value	7,477	15,948
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,008	73,028
Gross Unrealized Gain	18	2
Gross Unrealized Loss	(77)	(260)
Fair Value	$ 63,949	$ 72,770

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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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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