Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - 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 (loss) income	$ (1,397)	$ 15,402	$ (4,139)	$ (37,873)
Net derivative gains (losses) arising during the period, net of tax of $(559), $1,193, $(2,881), and $(1,894), respectively	1,488	(1,802)	6,303	2,536
Reclassification of losses into net loss, net of tax of $688, $1,209, $2,330, and $3,450, respectively	1,827	1,828	5,008	5,180
Other comprehensive income	3,315	26	11,311	7,716
Comprehensive income (loss)	$ 1,918	$ 15,428	$ 7,172	$ (30,157)