Revenue and Accounts Receivable - Rollforward of Allowance for Doubtful Accounts (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2023 USD ($) state	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)
Revenue from Contract with Customer [Abstract]
Number of states excluded due to gas cost recovery mechanisms | state	5
Percent of customers excluded due to gas cost recovery mechanisms	80.00%
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 49,993	$ 64,471	$ 29,949
Current period provisions	22,353	16,576	43,807
Write-offs charged against allowance	(33,595)	(32,885)	(11,019)
Recoveries of amounts previously written off	2,089	1,831	1,734
Ending balance	$ 40,840	$ 49,993	$ 64,471