Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 25.2	$ (2.4)	$ 19.5	$ (35.5)
Net derivative gains (losses) arising during the period, net of tax of $(0.3); $0.2; $(1.0); and $3.8, respectively	0.7	(0.5)	1.9	(10.3)
Reclassification of (gains) losses into net loss, net of tax of $0.1; $(1.9); $(1.4); and $(3.5), respectively	(0.3)	4.7	3.9	9.6
Other comprehensive (loss) income	0.4	4.2	5.8	(0.7)
Comprehensive (loss) income	$ 25.6	$ 1.8	$ 25.3	$ (36.2)