Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
Statement Of Financial Position [Abstract]
Accounts receivable, allowances	$ 21.0	$ 17.9	$ 14.3
Convertible preferred stock, par value	$ 0.01	$ 0.01	$ 0.01
Convertible preferred stock. aggregate liquidation preference	$ 400.0	$ 400.0	$ 400.0
Convertible preferred stock, shares authorized	400,000	400,000	400,000
Convertible preferred stock, shares issued	400,000	400,000	400,000
Convertible preferred stock, shares outstanding	400,000	400,000	400,000
Common stock (voting), par value	$ 0.01	$ 0.01	$ 0.01
Common stock (voting), shares authorized	100,000,000.0	100,000,000.0	100,000,000.0
Common Stock (voting), issued	69,800,000	69,000,000.0	68,800,000
Common Stock (voting), outstanding	69,800,000	69,000,000.0	68,800,000
Undesignated Preferred Stock, shares authorized	5,000,000.0	5,000,000.0	5,000,000.0
Undesignated Preferred Stock, issued	0	0	0
Undesignated Preferred Stock, outstanding	0	0	0