Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Accounts receivable, allowances	$ 13,470	$ 11,829	$ 16,711
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	67,972,383	67,700,858	60,030,516
Common Stock (voting), outstanding	67,972,383	67,700,858	60,030,516
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