Revenue and Accounts Receivable - Rollforward of Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Revenue [Abstract]
Beginning balance	$ 29,949	$ 15,899	$ 14,795
Current period provisions	43,807	24,796	18,106
Write-offs charged against allowance	(11,019)	(12,698)	(19,575)
Recoveries of amounts previously written off	1,734	1,952	2,573
Ending balance	$ 64,471	$ 29,949	$ 15,899