Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (36,168)	$ 46,383	$ 71,956
Other comprehensive income (loss):
Net unrealized gain (loss) on marketable securities, net of tax expense (benefit) of $388, $173, and ($109)	698	544	(286)
Total other comprehensive income (loss), net of tax	698	544	(286)
Comprehensive income (loss)	$ (35,470)	$ 46,927	$ 71,670