Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Statement Of Financial Position [Abstract]
Accounts receivable, allowances	$ 19.2	$ 13.1
Convertible preferred stock, par value	$ 0.01	$ 0.01
Convertible preferred stock. aggregate liquidation preference	$ 400.0	$ 400.0
Convertible preferred stock, shares authorized	400,000	400,000
Convertible preferred stock, shares issued	400,000	400,000
Convertible preferred stock, shares outstanding	400,000	400,000
Common stock (voting), par value	$ 0.01	$ 0.01
Common stock (voting), shares authorized	100,000,000.0	100,000,000.0
Common Stock (voting), issued	69,000,000.0	68,600,000
Common Stock (voting), outstanding	69,000,000.0	68,600,000
Undesignated Preferred Stock, shares authorized	5,000,000.0	5,000,000.0
Undesignated Preferred Stock, issued	0	0
Undesignated Preferred Stock, outstanding	0	0