Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (2.4)	$ 31.7	$ (35.5)	$ 19.3
Net derivative (losses) arising during the period, net of tax benefit of $0.2, $0.9, $3.8, and $2.3, respectively	(0.5)	(2.3)	(10.3)	(6.1)
Reclassification of losses into net (loss) income, net of tax expense of $1.9, $0.4, $3.5, and $1.4, respectively	4.7	1.2	9.6	3.6
Other comprehensive income (loss)	4.2	(1.1)	(0.7)	(2.5)
Comprehensive income (loss)	$ 1.8	$ 30.6	$ (36.2)	$ 16.8