Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018
Statement Of Financial Position [Abstract]
Accounts receivable, allowances	$ 24,732	$ 17,584	$ 13,963
Convertible preferred stock, par value	$ 0.01	$ 0.01	$ 0.01
Convertible preferred stock. aggregate liquidation preference	$ 400,000	$ 400,000	$ 400,000
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	100,000,000	100,000,000
Common Stock (voting), issued	68,480,751	68,135,790	68,105,113
Common Stock (voting), outstanding	68,480,751	68,135,790	68,105,113
Undesignated Preferred Stock, shares authorized	5,000,000	5,000,000	5,000,000
Undesignated Preferred Stock, issued	0	0	0
Undesignated Preferred Stock, outstanding	0	0	0