Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 6.3	$ (20.5)	$ (5.7)	$ (33.1)
Net derivative gains (losses) arising during the period, net of tax expense (benefit) of $0.5; $(3.7); $0.5; and $(3.7), respectively	1.6	(9.9)	1.3	(9.7)
Reclassification of losses into Net income (loss), net of tax expense of $0.1; $1.0; $1.5; and $1.8, respectively	0.2	2.9	4.1	4.8
Other comprehensive income (loss)	1.8	(7.0)	5.4	(4.9)
Comprehensive income (loss)	$ 8.1	$ (27.5)	$ (0.3)	$ (38.0)