CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)		$ (46)	$ (23,098)	$ 86,117	$ 51,542
Other comprehensive income (loss):
Unrealized gain on pension plan obligation	[1]	455	455	1,365	1,366
Tax effect of unrealized gain on pension plan obligation		(127)	(124)	(385)	(373)
Unrealized gain on captive insurance collateral		986	666	1,029	1,960
Tax effect of unrealized gain on captive insurance collateral		(207)	(140)	(214)	(416)
Unrealized loss on interest rate hedges		(126)	(733)	(1,508)	(1,845)
Tax effect of unrealized loss on interest rate hedges		34	196	408	488
Total other comprehensive income		1,015	320	695	1,180
Total comprehensive income (loss)		$ 969	$ (22,778)	$ 86,812	$ 52,722

[1] This item is included in the computation of net periodic pension cost.