Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (80.9)	$ (10.6)	$ 98.6
Other comprehensive income (loss):
Foreign currency translation adjustment	(0.7)	(1.7)	(2.7)
Unrealized gain (loss) due to change in fair value of derivatives, net of tax	(13.4)	(1.6)	0.0
Total other comprehensive income (loss)	(14.1)	(3.3)	(2.7)
Comprehensive income (loss)	$ (95.0)	$ (13.9)	$ 95.9