Supplementary Financial Information - Summary of Activity in Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 159	$ 172	$ 454
Provision	44	86	200
Write offs	(2)	(26)	(402)
Adjustment	(13)	(73)	(80)
Balance at end of year	$ 188	$ 159	$ 172