Revenue - Contract Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Contract assets	$ 47,886	$ 50,995
Contract liabilities	(70,939)	(79,445)
Net contract liability	(23,053)	(28,450)
Revenue recognized related to contract liabilities	36,400
Retention amounts included in accounts receivable	7,800	$ 6,900
Retained amount expected to be collected in the next twelve months	6,100
Retained amount expected to be collected	$ 1,700