Accounts Receivable - Movement in the allowance for doubtful accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ (4,906)	$ (4,199)	$ (3,494)
Charged to costs and expenses	(537)	(788)	(1,832)
Write-offs	1,277	39	1,387
Foreign currency and other	109	42	(260)
Balance at end of period	$ (4,057)	$ (4,906)	$ (4,199)