Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 297,368	$ 285,312
Gross Unrealized Gains	207	12
Gross Unrealized Losses	(357)	(802)
Fair Value	297,218	284,522
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,043	27,127
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(337)	(692)
Fair Value	26,706	26,435
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,975	21,305
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	5,975	21,305
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	264,350	236,880
Gross Unrealized Gains	207	12
Gross Unrealized Losses	(20)	(110)
Fair Value	$ 264,537	$ 236,782