Investments - Schedule of Maturities of Investments (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Due within one year		$ 4,505
Due after one year through five years		21,710
Due after five years through ten years		19,204
Total	[1]	$ 45,419	$ 11,777

[1] See Note 2 - Investments.