CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income		$ 72,325	$ 54,778	$ 74,640	$ 84,960
Other comprehensive income:
Unrealized gain on pension plan obligation	[1]	457	448	911	896
Tax effect of unrealized gain on pension plan obligation		(125)	(167)	(249)	(302)
Unrealized gain (loss) on captive insurance collateral		905	(931)	1,294	(931)
Tax effect of unrealized gain (loss) on captive insurance collateral		(194)	196	(276)	196
Unrealized loss on interest rate hedges		(367)		(1,112)
Tax effect of unrealized loss on interest rate hedges		95		292
Total other comprehensive income (loss)		771	(454)	860	(141)
Total comprehensive income		$ 73,096	$ 54,324	$ 75,500	$ 84,819

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