Acquisitions and Divestitures - Goodwill and Pro forma information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Unaudited pro forma information
Net sales	$ 318,800	$ 333,100
Net loss	(13,000)	$ (28,800)
Adjustments made for transaction expenses	0
Changes in the carrying amount of goodwill
Net balances at the beginning of the period	784,882
Acquisitions	(260)
Foreign currency exchange rate changes	3,119
Net balance at the end of the period	787,741
Impairment of goodwill	0
Cubic Transportation Systems
Changes in the carrying amount of goodwill
Net balances at the beginning of the period	287,668
Acquisitions	(260)
Foreign currency exchange rate changes	2,229
Net balance at the end of the period	289,637
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	890
Net balance at the end of the period	$ 498,104