CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 55,918	$ 17,637	$ 55,505
Other comprehensive income:
Unrealized gain on pension plan obligation	2,876	1,170	2,075
Tax effect of unrealized gain on pension plan obligation	(782)	(320)	(625)
Unrealized gain (loss) on captive insurance collateral	916	2,231	(1,204)
Tax effect of unrealized gain (loss) on captive insurance collateral	(190)	(473)	254
Unrealized gain (loss) on interest rate hedge	(1,193)	(1,993)	39
Tax effect of unrealized gain (loss) on interest rate hedge	317	525	(10)
Total other comprehensive income	1,944	1,140	529
Total comprehensive income	$ 57,862	$ 18,777	$ 56,034