Revenue - Contract Balances (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Contract assets	$ 42,728	$ 50,995
Contract liabilities	(54,098)	(79,445)
Net contract liability	(11,370)	(28,450)
Revenue recognized related to contract liabilities	55,200
Retention amounts included in accounts receivable	8,800	$ 6,900
Retained amount expected to be collected in the next twelve months	7,100
Retained amount expected to be collected	$ 1,700