Cash Equivalents and Investments - Amortized Cost and Fair Value of Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 229,997	$ 175,124
Gross Unrealized Gains	38	0
Gross Unrealized Losses	(42)	(198)
Available-for-sale Securities	229,993	174,926
Money Market Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	71,368	22,073
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale Securities	71,368	22,073
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	35,798	23,013
Gross Unrealized Gains	12	0
Gross Unrealized Losses	(1)	(13)
Available-for-sale Securities	35,809	23,000
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,638	45,683
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale Securities	49,638	45,683
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,147	29,127
Gross Unrealized Gains	6	0
Gross Unrealized Losses	0	(34)
Available-for-sale Securities	27,153	29,093
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,046	55,228
Gross Unrealized Gains	20	0
Gross Unrealized Losses	(41)	(151)
Available-for-sale Securities	$ 46,025	$ 55,077

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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-Publisher FASB
-Name Accounting Standards Codification
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