Cash Equivalents, Restricted Cash and Investments - Amortized Cost and Fair Value of Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 499,467	$ 193,068
Gross Unrealized Gains	490	322
Gross Unrealized Losses	(2)	(4)
Available-for-sale Securities	499,955	193,386
Money Market Fund [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	328,540	30,353
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale Securities	328,540	30,353
U.S. Treasury Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	58,174	40,245
Gross Unrealized Gains	146	47
Gross Unrealized Losses	(1)	0
Available-for-sale Securities	58,319	40,292
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	50,316	12,429
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale Securities	50,316	12,429
Asset-backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	15,080	25,436
Gross Unrealized Gains	114	50
Gross Unrealized Losses	0	0
Available-for-sale Securities	15,194	25,486
Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	47,357	84,605
Gross Unrealized Gains	230	225
Gross Unrealized Losses	(1)	(4)
Available-for-sale Securities	$ 47,586	$ 84,826

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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