Investments - Schedule of Maturities of Investments (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Due within one year		$ 4,455
Due after one year through five years		28,902
Due after five years through ten years		16,896
Total	[1]	$ 50,253	$ 45,419

[1] See Note 2 – Investments