Revenue - Contract Balances (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2020	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Contract assets	$ 61,336	$ 61,336	$ 55,374
Contract liabilities	(57,821)	(57,821)	(71,464)
Net contract asset (liability)	3,515	3,515	(16,090)
Revenue recognized related to contract liabilities	50,800	65,000
Retention amounts included in accounts receivable	6,000	6,000	$ 5,600
Retained amount expected to be collected in the next twelve months	3,700	3,700
Retained amount expected to be collected	$ 2,300	$ 2,300