Goodwill and Purchased Intangible Assets - Goodwill (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) item
Changes in the carrying amount of goodwill
Balance at the beginning of the period	$ 406,946	$ 237,899
Acquisitions	5,885	175,150
Foreign currency exchange rate changes	3,081	(6,103)
Balance at the end of the period	415,912	406,946
Components of net goodwill
Goodwill			$ 466,777
Accumulated impairment charges			(50,865)
Net balances	406,946	237,899	$ 415,912
Number of remaining reporting units whose estimated fair values exceeded carrying values | item			3
Impairment of goodwill	0
Transportation Systems
Changes in the carrying amount of goodwill
Balance at the beginning of the period	49,630	55,974
Foreign currency exchange rate changes	1,240	(6,344)
Balance at the end of the period	50,870	49,630
Components of net goodwill
Goodwill			$ 50,870
Net balances	$ 49,630	55,974	50,870
Transportation Systems | Minimum
Components of net goodwill
Estimated percentage of excess of fair value over carrying value	100.00%
Defense Systems
Changes in the carrying amount of goodwill
Balance at the beginning of the period	$ 262,966	87,575
Acquisitions	5,885	175,150
Foreign currency exchange rate changes	1,841	241
Balance at the end of the period	270,692	262,966
Components of net goodwill
Goodwill			270,692
Net balances	$ 262,966	87,575	270,692
Defense Systems | Minimum
Components of net goodwill
Estimated percentage of excess of fair value over carrying value	10.00%
Cubic Global Defense Services
Changes in the carrying amount of goodwill
Balance at the beginning of the period	$ 94,350	94,350
Acquisitions
Foreign currency exchange rate changes
Balance at the end of the period	94,350	94,350
Components of net goodwill
Goodwill			145,215
Accumulated impairment charges			(50,865)
Net balances	$ 94,350	$ 94,350	$ 94,350
Cubic Global Defense Services | Minimum
Components of net goodwill
Estimated percentage of excess of fair value over carrying value	10.00%