Goodwill and Purchased Intangible Assets - Goodwill (Details) $ in Thousands		12 Months Ended
	Apr. 01, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)
Changes in the carrying amount of goodwill
Net balances at the beginning of the period		$ 578,097	$ 333,626
Acquisitions		204,259	247,380
Foreign currency exchange rate changes		2,526	(2,909)
Net balance at the end of the period		$ 784,882	578,097
Number of reporting segments | segment		3
Impairment of goodwill		$ 0
Cubic Transportation Systems
Changes in the carrying amount of goodwill
Net balances at the beginning of the period		254,592	49,786
Acquisitions		31,224	206,988
Foreign currency exchange rate changes		1,852	(2,182)
Net balance at the end of the period		287,668	254,592
Cubic Mission Solutions
Changes in the carrying amount of goodwill
Net balances at the beginning of the period		181,424	138,127
Reassignment of goodwill	$ 3,400		3,428
Acquisitions		173,035	40,392
Foreign currency exchange rate changes		478	(523)
Net balance at the end of the period		354,937	181,424
Cubic Global Defense Systems
Changes in the carrying amount of goodwill
Net balances at the beginning of the period		142,081	145,713
Reassignment of goodwill	$ (3,400)		(3,428)
Foreign currency exchange rate changes		196	(204)
Net balance at the end of the period		$ 142,277	$ 142,081