Cash Equivalents, Restricted Cash and Investments - Amortized Cost and Fair Value of Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 193,068	$ 175,124
Gross Unrealized Gains	322	0
Gross Unrealized Losses	(4)	(198)
Available-for-sale Securities	193,386	174,926
Money Market Fund [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	30,353	22,073
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale Securities	30,353	22,073
U.S. Treasury Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	40,245	23,013
Gross Unrealized Gains	47	0
Gross Unrealized Losses	0	(13)
Available-for-sale Securities	40,292	23,000
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,429	45,683
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale Securities	12,429	45,683
Asset-backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	25,436	29,127
Gross Unrealized Gains	50	0
Gross Unrealized Losses	0	(34)
Available-for-sale Securities	25,486	29,093
Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	84,605	55,228
Gross Unrealized Gains	225	0
Gross Unrealized Losses	(4)	(151)
Available-for-sale Securities	$ 84,826	$ 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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