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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, 2020
Sep. 30, 2019
Assets:    
Marketable securities $ 335,549 $ 349,019
Assets, Fair Value Disclosure, Total 423,157 393,588
Liabilities:    
Liabilities, Fair Value Disclosure, Total 1,628 1,628
Series 1 Nonconvertible Preferred Stock [Member]    
Liabilities:    
Liabilities 1,628 1,628
Level 1 [Member]    
Assets:    
Assets, Fair Value Disclosure, Total 227,510 215,084
Level 2 [Member]    
Assets:    
Assets, Fair Value Disclosure, Total 195,647 178,504
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
Money Market Funds [Member]    
Assets:    
Cash equivalents 87,608 44,569
Money Market Funds [Member] | Level 1 [Member]    
Assets:    
Cash equivalents 87,608 44,569
Corporate Bonds [Member]    
Assets:    
Marketable securities 137,823 111,837
Corporate Bonds [Member] | Level 2 [Member]    
Assets:    
Marketable securities 137,823 111,837
Commercial Paper [Member]    
Assets:    
Marketable securities 57,824 66,667
Commercial Paper [Member] | Level 2 [Member]    
Assets:    
Marketable securities 57,824 66,667
U.S. Treasury Notes [Member]    
Assets:    
Marketable securities 139,902 170,515
U.S. Treasury Notes [Member] | Level 1 [Member]    
Assets:    
Marketable securities $ 139,902 $ 170,515