Details of Selected Consolidated Balance Sheet Captions - Accounts Receivables Detail Table (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 260,544	$ 246,470
Less: allowance for doubtful accounts	(29,949)	(15,899)
Net accounts receivable	230,595	230,571
Billed and Unbilled Receivables | Billed accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	140,259	126,984
Billed and Unbilled Receivables | Unbilled revenue
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	80,699	78,986
Contributions in aid of construction receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	19,821	22,378
Other accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 19,765	$ 18,122