Fair Value of Financial Instruments - Contingent Consideration (Details) $ in Thousands	1 Months Ended	9 Months Ended	12 Months Ended
	Jul. 31, 2018 USD ($) item	Jun. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($) item	Mar. 31, 2019 USD ($)
Cubic Global Defense Services | Disposed of by sale
Assets and liabilities measured at fair value on a recurring basis
Receivable for estimated earn-out amount due from purchaser		$ 3,000
Level 3
Change in fair value of our Level 3 contingent consideration
Balance at the Beginning period		9,114
Initial measurement recognized at acquisition		4,900
Cash paid to seller		(2,135)
Total remeasurement (gain) loss recognized in earnings		1,833
Balance at the ending period		13,712	$ 9,114
Investment Assets | Level 3
Assets and liabilities measured at fair value on a recurring basis
Maximum additional commitment to fund		6,900
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Cash paid for initial investment		3,100
Total remeasurement gain/(loss) recognized in earnings		3,076
Balance as of June 30, 2019		$ 3,076
Volatility for underlying revenue metrics
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs for contingent consideration | item		12.9
Deltenna | Level 3
Change in fair value of our Level 3 contingent consideration
Balance at the Beginning period		$ 1,081
Total remeasurement (gain) loss recognized in earnings		1,499
Balance at the ending period		2,580	$ 1,081
Deltenna | Maximum
Assets and liabilities measured at fair value on a recurring basis
Contingent consideration		$ 7,100
Deltenna | Volatility for underlying revenue metrics
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs for contingent consideration | item		37	53
Deltenna | Discount Rate
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs for contingent consideration		11	11.5
Shield Aviation
Assets and liabilities measured at fair value on a recurring basis
Contingent consideration	$ 5,600
Determination of fair value of contingent consideration | item	1,000,000
Revenue risk adjustment used in determination of fair value of contingent consideration		17.40%
Shield Aviation | Level 3
Change in fair value of our Level 3 contingent consideration
Balance at the Beginning period		$ 5,618
Total remeasurement (gain) loss recognized in earnings		(415)
Balance at the ending period		5,203	$ 5,618
Shield Aviation | Maximum
Assets and liabilities measured at fair value on a recurring basis
Contingent consideration	$ 10,000	$ 10,000
Shield Aviation | Volatility for underlying revenue metrics
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs for contingent consideration		0.260	0.200
Revenue risk adjustment used in determination of fair value of contingent consideration			14.50%
Shield Aviation | Discount Rate
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs for contingent consideration		0.041	0.039
TeraLogics | Level 3 | Revenue Targets
Change in fair value of our Level 3 contingent consideration
Balance at the Beginning period		$ 1,750
Cash paid to seller		$ (1,750)
Balance at the ending period			$ 1,750
H4 Global
Assets and liabilities measured at fair value on a recurring basis
Period for contingent consideration contracts		5 years
H4 Global | Level 3
Change in fair value of our Level 3 contingent consideration
Balance at the Beginning period		$ 665
Cash paid to seller		(385)
Total remeasurement (gain) loss recognized in earnings		399
Balance at the ending period		679	$ 665
H4 Global | Maximum
Assets and liabilities measured at fair value on a recurring basis
Contingent consideration		$ 3,000
Nuvotronics
Assets and liabilities measured at fair value on a recurring basis
Contingent consideration				$ 4,900
Determination of fair value of contingent consideration | item		1,000,000
Period for contingent consideration contracts		12 months
Nuvotronics | Level 3
Change in fair value of our Level 3 contingent consideration
Initial measurement recognized at acquisition		$ 4,900
Total remeasurement (gain) loss recognized in earnings		350
Balance at the ending period		5,250
Nuvotronics | Maximum
Assets and liabilities measured at fair value on a recurring basis
Contingent consideration		$ 8,000		$ 8,000
Nuvotronics | Discount Rate
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs for contingent consideration		7.3