Goodwill and Purchased Intangible Assets - Goodwill (Details) $ in Thousands		12 Months Ended
Apr. 01, 2019 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)
Changes in the carrying amount of goodwill
Net balances at the beginning of the period		$ 333,626	$ 321,562
Acquisitions		247,380	13,750
Foreign currency exchange rate changes		(2,909)	(1,686)
Net balance at the end of the period		$ 578,097	333,626
Number of reporting segments \| item		3
Impairment of goodwill		$ 0
Cubic Transportation Systems
Changes in the carrying amount of goodwill
Net balances at the beginning of the period		49,786	50,870
Acquisitions		206,988
Foreign currency exchange rate changes		(2,182)	(1,084)
Net balance at the end of the period		254,592	49,786
Cubic Mission Solutions
Changes in the carrying amount of goodwill
Net balances at the beginning of the period		138,127
Reassignment of goodwill	$ 3,428		125,321
Acquisitions		40,392	13,085
Foreign currency exchange rate changes		(523)	(279)
Net balance at the end of the period		181,424	138,127
Cubic Global Defense
Changes in the carrying amount of goodwill
Net balances at the beginning of the period		145,713	270,692
Reassignment of goodwill	(3,428)		(125,321)
Acquisitions			665
Foreign currency exchange rate changes		(204)	(323)
Net balance at the end of the period		$ 142,081	$ 145,713
Deltenna \| Cubic Mission Solutions
Changes in the carrying amount of goodwill
Reassignment of goodwill	3,400
Deltenna \| Cubic Global Defense
Changes in the carrying amount of goodwill
Reassignment of goodwill	$ (3,400)

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of transfers into (out of) an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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