Revenue - Contract Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Contract assets	$ 54,199	$ 50,995
Contract liabilities	(42,433)	(79,445)
Net contract asset (liability)	11,766	(28,450)
Revenue recognized related to contract liabilities	69,100
Retention amounts included in accounts receivable	9,600	$ 6,900
Retained amount expected to be collected in the next twelve months	7,400
Retained amount expected to be collected	$ 2,200