Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 46,383	$ 71,956	$ 17,710
Other comprehensive income (loss):
Net unrealized gain (loss) on marketable securities, net of tax expense (benefit) of $173, ($109), and ($78)	544	(286)	(131)
Total other comprehensive income (loss), net of tax	544	(286)	(131)
Comprehensive income	$ 46,927	$ 71,670	$ 17,579