Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Jun. 30, 2023	Sep. 30, 2022
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 298,452	$ 237,863
Gross Unrealized Gains	22	0
Gross Unrealized Losses	(1,142)	(3,340)
Fair Value	297,332	234,523
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,210	78,663
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(954)	(2,252)
Fair Value	36,256	76,411
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	194,427	92,416
Gross Unrealized Gains	22	0
Gross Unrealized Losses	(188)	(1,088)
Fair Value	194,261	91,328
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	66,815	66,784
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 66,815	$ 66,784