Contract Assets and Liabilities - Cost and Estimated Earnings Compared to Billings on Uncompleted Contracts (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Costs on uncompleted contracts	$ 970,140	$ 900,880
Estimated earnings to date on uncompleted contracts	116,748	123,256
Costs and estimated earnings to date on uncompleted contracts	1,086,888	1,024,136
Billings to date on uncompleted contracts	(1,105,795)	(1,043,221)
Net billings in excess of costs and estimated earnings on uncompleted contracts	$ (18,907)	$ (19,085)