Marketable Securities - Available-For-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 291,138	$ 266,750
Gross Unrealized Gains	446	313
Gross Unrealized Losses	(9)	(6)
Fair Market Value	291,575	267,057
U.S. Treasury and government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	151,307	88,535
Gross Unrealized Gains	197	68
Gross Unrealized Losses	(3)	(2)
Fair Market Value	151,501	88,601
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	139,831	129,860
Gross Unrealized Gains	249	196
Gross Unrealized Losses	(6)	(1)
Fair Market Value	$ 140,074	130,055
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		48,355
Gross Unrealized Gains		49
Gross Unrealized Losses		(3)
Fair Market Value		$ 48,401

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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