CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)		$ (8,005)	$ (13,332)	$ 76,955	$ 44,647
Other comprehensive income:
Unrealized gain on pension plan obligation	[1]	448	532	1,344	1,598
Tax effect of unrealized gain on pension plan		(120)	(215)	(422)	(647)
Unrealized loss on captive insurance collateral		(220)		(1,151)
Tax effect of unrealized loss on captive insurance collateral		46		242
Total other comprehensive income		154	317	13	951
Total comprehensive income (loss)		$ (7,851)	$ (13,015)	$ 76,968	$ 45,598

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