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, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Assets:
Assets, Fair Value Disclosure, Total	$ 393,588	$ 323,227
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,628	1,628
Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,628	1,628	$ 762	$ 159
Level 1 [Member]
Assets:
Assets, Fair Value Disclosure, Total	215,084	93,728
Level 2 [Member]
Assets:
Assets, Fair Value Disclosure, Total	178,504	229,499
Level 3 [Member]
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,628	1,628
Level 3 [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,628	1,628
U.S. Treasury Notes [Member]
Assets:
Marketable securities	170,515	42,703
U.S. Treasury Notes [Member] | Level 1 [Member]
Assets:
Marketable securities	170,515	42,703
Money Market Funds [Member]
Assets:
Cash equivalents	44,569	51,025
Money Market Funds [Member] | Level 1 [Member]
Assets:
Cash equivalents	44,569	51,025
Commercial Paper [Member]
Assets:
Cash equivalents		6,987
Marketable securities	66,667	113,885
Commercial Paper [Member] | Level 2 [Member]
Assets:
Cash equivalents		6,987
Marketable securities	66,667	113,885
Corporate Bonds [Member]
Assets:
Cash equivalents		3,998
Marketable securities	111,837	104,629
Corporate Bonds [Member] | Level 2 [Member]
Assets:
Cash equivalents		3,998
Marketable securities	$ 111,837	$ 104,629