CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 2,718	$ 2,377
Prepaid debt issuance cost	$ 16,420	$ 17,900
Common stock, authorized (in shares)	200,000,000	200,000,000
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, issued (in shares)	39,854,529	39,592,468
Treasury stock (in shares)	10,672,652	10,491,252