Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 330,923	$ 348,876
Gross Unrealized Gains	1,243	230
Gross Unrealized Losses	(14)	(87)
Marketable securities	332,152	349,019
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	143,274	111,690
Gross Unrealized Gains	775	170
Gross Unrealized Losses	(14)	(23)
Marketable securities	144,035	111,837
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	93,740	170,519
Gross Unrealized Gains	468	60
Gross Unrealized Losses		(64)
Marketable securities	94,208	170,515
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	93,909	66,667
Marketable securities	$ 93,909	$ 66,667

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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