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	Jun. 30, 2022	Sep. 30, 2021
Assets:
Marketable securities	$ 264,348	$ 295,212
Assets, Fair Value Disclosure, Total	277,902	350,031
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,506	1,506
Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,506	1,506
Level 1 [Member]
Assets:
Assets, Fair Value Disclosure, Total	110,730	137,857
Level 2 [Member]
Assets:
Assets, Fair Value Disclosure, Total	167,172	212,174
Level 3 [Member]
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,506	1,506
Level 3 [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,506	1,506
Money Market Funds [Member]
Assets:
Cash equivalents	13,554
Money Market Funds [Member] | Level 1 [Member]
Assets:
Cash equivalents	13,554	54,819
Corporate Bonds [Member]
Assets:
Marketable securities	83,947	124,703
Corporate Bonds [Member] | Level 2 [Member]
Assets:
Marketable securities	83,947	124,703
Commercial Paper [Member]
Assets:
Marketable securities	83,225	87,471
Commercial Paper [Member] | Level 2 [Member]
Assets:
Marketable securities	83,225	87,471
U.S. Treasury Notes [Member]
Assets:
Marketable securities	97,176	83,038
U.S. Treasury Notes [Member] | Level 1 [Member]
Assets:
Marketable securities	$ 97,176	$ 83,038