Segment Information - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Revenue recognition
Increase (decrease) in operating income	$ (1,679)	$ (6,084)	$ (13,581)	$ (11,707)
Net income (loss) from continuing operations	$ (3,346)	$ 41,871	$ (14,781)	$ 42,887
Increase (decrease) in net income (loss) from continuing operations per common share (in dollars per share)	$ (0.12)	$ 1.54	$ (0.54)	$ 1.58
P3 Joint Venture
Revenue recognition
Ownership percentage	10.00%		10.00%
Expected contract revenue	$ 664,000		$ 664,000
Contract term			10 years
Expected contract payments	510,000		$ 510,000
Sub contract period			3 years
Maximum borrowing capacity under credit agreement	212,400		$ 212,400
Debt instrument term			10 years
Variable interest rate (as a percent)			1.30%
Proceeds from issuance of debt			$ 3,300
Deferred financing costs	$ 9,000		$ 9,000
P3 Joint Venture \| John Laing
Revenue recognition
Ownership percentage	90.00%		90.00%
P3 Joint Venture \| John Laing \| Equity Bridge Loan
Revenue recognition
Loan amount	$ 24,300		$ 24,300
Interest rate (as a percent)	2.50%		2.50%
P3 Joint Venture \| John Laing \| Letter of credit agreement
Revenue recognition
Maximum borrowing capacity under credit agreement	$ 2,700		$ 2,700
Line of credit	0		0
Cubic Mission Solutions
Revenue recognition
Increase (decrease) in operating income	(7,800)	$ (9,200)	(16,700)	$ (13,000)
Transportation Systems
Revenue recognition
Increase (decrease) in operating income	14,200	7,800	24,100	17,500
Other current assets \| Cubic Mission Solutions
Revenue recognition
Loan receivables	3,700		3,700
Other noncurrent assets \| Cubic Mission Solutions
Revenue recognition
Carrying value of warrants	700		700
Unallocated corporate expenses and other
Revenue recognition
Expense related to the development of ERP system	5,700	6,000	13,700	14,600
unallocated corporate expenses	1,900	2,000	3,900	3,900
Increase (decrease) in operating income	(13,400)	(9,500)	(27,700)	(24,300)
Change in estimated total costs \| Adjustment
Revenue recognition
Increase (decrease) in operating income	(300)	(3,400)	(2,200)	1,300
Net income (loss) from continuing operations	$ 300	$ 2,300	$ (1,600)	$ (600)
Increase (decrease) in net income (loss) from continuing operations per common share (in dollars per share)	$ 0.01	$ 0.09	$ (0.06)	$ (0.02)

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