Contract Balances and Performance Obligations - Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accounts Receivable, after Allowance for Credit Loss [Abstract]
Billed Receivables	$ 211,776	$ 206,714
Unbilled Receivables	181,550	127,651
Receivables Gross	393,326	334,365
Allowance for doubtful accounts	(5,072)	(2,568)	$ (3,439)
Net receivables	388,254	331,797
Accounts Receivable, net , noncurrent	(54,074)	(34,370)
Accounts receivable, net	334,180	297,427
Expense, Allowance for Credit Loss	3,199	518
Write-off	$ (695)	$ (1,389)