Details of Selected Financial Statement Captions - Accounts Receivables (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 369,494	$ 413,701
Less: allowance for uncollectible accounts	(40,840)	(49,993)
Net accounts receivable	328,654	363,708
Billed and Unbilled Receivables | Billed accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	198,976	258,333
Billed and Unbilled Receivables | Unbilled revenue
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	105,743	121,518
Contributions in aid of construction receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	17,184	5,390
Insurance receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	33,697	13,160
Other accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 13,894	$ 15,300