Details of Selected Consolidated Balance Sheet Captions - Accounts Receivables (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 413,701	$ 407,438
Less: allowance for uncollectible accounts	(49,993)	(64,471)	$ (29,949)	$ (15,899)
Net accounts receivable	363,708	342,967
Billed and Unbilled Receivables | Billed accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	258,333	218,219
Billed and Unbilled Receivables | Unbilled revenue
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	121,518	97,417
Contributions in aid of construction receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	5,390	18,984
Insurance receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	13,160	53,779
Other accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 15,300	$ 19,039