Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Statement Of Financial Position [Abstract]
Accounts receivable, allowances	$ 23,058	$ 17,584	$ 16,493
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	68,475,871	68,135,790	68,043,284
Common Stock (voting), outstanding	68,475,871	68,135,790	68,043,284
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