Business Segment Information - Narrative (Details) $ / shares in Units, $ in Thousands	3 Months Ended									11 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares	Mar. 31, 2015 USD ($)	Aug. 31, 2015 USD ($)	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($) item $ / shares	Sep. 30, 2015 USD ($) contract $ / shares
Revenue recognition
Increase (decrease) in operating income	$ 25,384	$ (1,697)	$ (2,072)	$ (4,101)	$ 10,488	$ 13,893	$ (9,086)	$ (8,077)			$ 17,514	$ 7,218	$ 75,387
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest											$ (11,209)	$ 1,735	$ 22,914
Increase (decrease) in net income per common share (in dollars per share) | $ / shares	$ 0.49	$ (0.81)	$ 0.02	$ (0.11)	$ (0.29)	$ 0.17	$ 0.38	$ (0.20)			$ (0.41)	$ 0.06	$ 0.85
Restructuring costs											$ 2,468	$ 1,852	$ 6,272
Defense Systems
Revenue recognition
Increase (decrease) in operating income											18,800	(17,100)	18,400
Restructuring costs											900	300	4,600
Cubic Global Defense Services
Revenue recognition
Increase (decrease) in operating income											6,700	11,200	6,600
Restructuring costs											200	600	600
Transportation Systems
Revenue recognition
Increase (decrease) in operating income											39,800	57,500	$ 75,900
Number of transportation systems service contracts contains annual system usage incentives | contract													1
Sales related to annual system usage incentives									$ 9,300	$ 3,100
Restructuring costs											400	$ 1,000	$ 600
Reduction in employee headcount | item												20
CGD services and CGD systems segments
Revenue recognition
Restructuring costs												$ 900
Unallocated corporate expenses and other
Revenue recognition
Increase (decrease) in operating income											(47,800)	(44,400)	(25,500)
Restructuring costs											1,000		500
Change in estimated total costs | Adjustment
Revenue recognition
Increase (decrease) in operating income	$ (4,600)				$ 1,300						(100)	(2,800)	(14,500)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ (2,900)				$ 900						$ (300)	$ (1,600)	$ (8,000)
Increase (decrease) in net income per common share (in dollars per share) | $ / shares	$ (0.11)				$ 0.03						$ (0.01)	$ (0.06)	$ (0.30)