Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (14.0)	$ 44.4	$ (15.1)
Net derivative (losses) gains arising during the period, net of tax of $2.4, $(3.1), and $1.6, respectively	(2.4)	(6.4)	6.8
Reclassification of losses into net income (loss), net of tax of $2.0, $3.0, and $3.1, respectively	4.7	5.1	6.8
Other comprehensive (loss) income	2.3	(1.3)	13.6
Comprehensive income (loss)	$ (11.7)	$ 43.1	$ (1.5)