Supplementary Balance Sheet Information - Allowance for Expected Credit Losses Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 7,146	$ 3,548	$ 463
Provisions	3,445	4,600	3,327
Reversals of bad debt expense	(6,273)	(1,002)	(242)
Balance at end of period	$ 4,318	$ 7,146	$ 3,548