Fair Value Measurements and Marketable Securities - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 38,627	$ 68,767
Gross Unrealized Gains	42	3
Gross Unrealized Losses		(35)
Fair Value	38,669	68,735
Commercial Paper (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,479	38,921
Gross Unrealized Losses		(10)
Fair Value	13,479	38,911
Corporate Bonds (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,137	25,851
Gross Unrealized Gains	32	3
Gross Unrealized Losses		(24)
Fair Value	13,169	25,830
U.S. Government Securities (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,011	3,995
Gross Unrealized Gains	10
Gross Unrealized Losses		(1)
Fair Value	$ 12,021	$ 3,994

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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