Supplementary Financial Information - Summary of Activity in Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 172	$ 454	$ 356
Provision	86	200	102
Write offs	(26)	(402)	(9)
Adjustment	(73)	(80)	5
Balance at end of year	$ 159	$ 172	$ 454