Segment Information - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Revenue recognition
Increase (decrease) in operating income	$ (7,004)	$ (4,101)
Net loss	$ (9,786)	$ (2,868)
Increase (decrease) in net income per common share (in dollars per share)	$ (0.36)	$ (0.11)
Cubic Mission Solutions
Revenue recognition
Increase (decrease) in operating income	$ (8,900)	$ (3,800)
Note receivable	3,400
Unallocated corporate expenses and other
Revenue recognition
Expense related to the development of ERP system	8,100	8,700
Increase (decrease) in operating income	(12,300)	(12,900)
Change in estimated total costs | Adjustment
Revenue recognition
Increase (decrease) in operating income	(600)	(6,400)
Net loss	$ (500)	$ (3,900)
Increase (decrease) in net income per common share (in dollars per share)	$ (0.02)	$ (0.14)