Revenue - Contract Balances (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2021	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Contract assets	$ 50,827	$ 50,995
Contract liabilities	(43,140)	(79,445)
Net contract asset (liability)	7,687	(28,450)
Revenue recognized related to contract liabilities	63,500
Retention amounts included in accounts receivable	9,300	$ 6,900
Retained amount expected to be collected in the next twelve months	7,300
Retained amount expected to be collected	$ 2,000