CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income		$ 54,778	$ 39,704	$ 84,960	$ 57,979
Other comprehensive income (loss):
Unrealized gain on pension plan obligation (1)	[1]	448	532	896	1,066
Tax effect of unrealized gain on pension plan		(167)	(216)	(302)	(432)
Unrealized loss on captive insurance collateral		(931)		(931)
Tax effect of unrealized loss on captive insurance collateral		196		196
Total other comprehensive income (loss)		(454)	316	(141)	634
Total comprehensive income		$ 54,324	$ 40,020	$ 84,819	$ 58,613

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