Investments in Marketable Securities - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Amortized Cost	$ 60,451	$ 79,555
Gross Unrealized Gains	28	5
Gross Unrealized Losses	(1)	(22)
Fair Value	60,478	79,538
U.S. Treasury Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	10,981	17,028
Gross Unrealized Gains	7
Gross Unrealized Losses		(2)
Fair Value	10,988	17,026
Commercial Paper [Member]
Marketable Securities [Line Items]
Amortized Cost	35,545	48,623
Gross Unrealized Gains	8	5
Gross Unrealized Losses	(1)	(4)
Fair Value	35,552	48,624
Asset Backed Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	13,925	13,904
Gross Unrealized Gains	13
Gross Unrealized Losses		(16)
Fair Value	$ 13,938	$ 13,888

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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