Details of Selected Consolidated Balance Sheet Captions - Accounts Receivables Detail Table (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 246,470	$ 268,090
Less: allowance for doubtful accounts	15,899	14,795
Net accounts receivable	230,571	253,295
Billed and Unbilled Receivables | Billed accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	126,984	138,794
Billed and Unbilled Receivables | Unbilled revenue
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	78,986	81,005
Contributions in aid of construction receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	22,378	23,015
Other accounts receivable
Condensed Balance Sheet Statements, Captions [Line Items]
Total accounts receivable	$ 18,122	$ 25,276