Business Acquisitions - Goodwill and Pro forma information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Unaudited pro forma information
Net sales	$ 343,400	$ 321,500	$ 662,200	$ 654,600
Net loss	(36,000)	$ (39,300)	(49,000)	$ (68,100)
Adjustments made for transaction expenses			0
Changes in the carrying amount of goodwill
Net balances at the beginning of the period			784,882
Adjustments			(260)
Foreign currency exchange rate changes			3,405
Net balance at the end of the period	788,027		788,027
Impairment of goodwill			0
Cubic Transportation Systems
Changes in the carrying amount of goodwill
Net balances at the beginning of the period			287,668
Adjustments			(260)
Foreign currency exchange rate changes			2,541
Net balance at the end of the period	289,949		289,949
Cubic Mission Performance Solutions
Changes in the carrying amount of goodwill
Net balances at the beginning of the period			497,214
Foreign currency exchange rate changes			864
Net balance at the end of the period	$ 498,078		$ 498,078