Revenue - Contract Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Contract assets	$ 50,995	$ 55,374
Contract liabilities	(79,445)	(71,464)
Net contract liability	(28,450)	(16,090)
Revenue recognized related to contract liabilities	71,000
Retention amounts included in accounts receivable	6,900	$ 5,600
Retained amount expected to be collected in the next twelve months	5,900
Retained amount expected to be collected	$ 1,000