CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)		$ (23,098)	$ (8,005)	$ 51,542	$ 76,955
Other comprehensive income:
Unrealized gain on pension plan obligation	[1]	455	448	1,366	1,344
Tax effect of unrealized gain on pension plan obligation		(124)	(120)	(373)	(422)
Unrealized gain (loss) on captive insurance collateral		666	(220)	1,960	(1,151)
Tax effect of unrealized gain (loss) on captive insurance collateral		(140)	46	(416)	242
Unrealized loss on interest rate hedges		(733)		(1,845)
Tax effect of unrealized loss on interest rate hedges		196		488
Total other comprehensive income		320	154	1,180	13
Total comprehensive income (loss)		$ (22,778)	$ (7,851)	$ 52,722	$ 76,968

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