Accounts Receivable - Movement in the allowance for doubtful accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ (4,199)	$ (3,494)	$ (4,784)
Charged to costs and expenses	(788)	(1,832)	(1,206)
Write-offs	39	1,387	2,481
Foreign currency and other	42	(260)	15
Balance at end of period	$ (4,906)	$ (4,199)	$ (3,494)