Fair Value of Financial Assets and Liabilities - Financial Assets and Liabilities that were Subject to Fair Value Measurement on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2019
Assets:
Marketable securities	$ 284,522	$ 234,523
Assets, Fair Value Disclosure, Total	369,634	248,428
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,423	1,423
Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,423	1,423	$ 1,506	$ 1,479
Level 1 [Member]
Assets:
Assets, Fair Value Disclosure, Total	321,894	105,233
Liabilities:
Liabilities, Fair Value Disclosure, Total	0	0
Level 1 [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	0	0
Level 2 [Member]
Assets:
Assets, Fair Value Disclosure, Total	47,740	143,195
Liabilities:
Liabilities, Fair Value Disclosure, Total	0	0
Level 2 [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	0	0
Level 3 [Member]
Assets:
Assets, Fair Value Disclosure, Total	0	0
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,423	1,423
Level 3 [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,423	1,423
U.S. Treasury Notes [Member]
Assets:
Cash equivalents	29,755
Marketable securities	236,782	91,328
U.S. Treasury Notes [Member] | Level 1 [Member]
Assets:
Cash equivalents	29,755
Marketable securities	236,782	91,328
U.S. Treasury Notes [Member] | Level 2 [Member]
Assets:
Cash equivalents	0
Marketable securities	0	0
U.S. Treasury Notes [Member] | Level 3 [Member]
Assets:
Marketable securities	0	0
Money Market Funds [Member]
Assets:
Cash equivalents	55,357	13,905
Money Market Funds [Member] | Level 1 [Member]
Assets:
Cash equivalents	55,357	13,905
Money Market Funds [Member] | Level 2 [Member]
Assets:
Cash equivalents	0	0
Money Market Funds [Member] | Level 3 [Member]
Assets:
Cash equivalents	0	0
Commercial Paper [Member]
Assets:
Marketable securities	21,305	66,784
Commercial Paper [Member] | Level 1 [Member]
Assets:
Marketable securities	0	0
Commercial Paper [Member] | Level 2 [Member]
Assets:
Marketable securities	21,305	66,784
Commercial Paper [Member] | Level 3 [Member]
Assets:
Marketable securities	0	0
Corporate Bonds [Member]
Assets:
Marketable securities	26,435	76,411
Corporate Bonds [Member] | Level 1 [Member]
Assets:
Marketable securities	0	0
Corporate Bonds [Member] | Level 2 [Member]
Assets:
Marketable securities	26,435	76,411
Corporate Bonds [Member] | Level 3 [Member]
Assets:
Marketable securities	$ 0	$ 0