Details of Selected Consolidated Balance Sheet Captions - Accounts Receivables (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 407,438	$ 260,544
Less: allowance for uncollectible accounts	(64,471)	(29,949)	$ (15,899)	$ (14,795)
Net accounts receivable	342,967	230,595
Billed and Unbilled Receivables | Billed accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	218,219	140,259
Billed and Unbilled Receivables | Unbilled revenue
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	97,417	80,699
Contributions in aid of construction receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	18,984	19,821
Insurance receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	53,779	4,731
Other accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 19,039	$ 15,034