Acquisitions and Divestitures - Consolidated Business Acquisitions (Details) - USD ($) $ in Thousands		1 Months Ended				3 Months Ended		9 Months Ended
	Feb. 21, 2018	Mar. 31, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Oct. 01, 2018	Sep. 30, 2018
Sales and results of operations
Sales						$ 382,679	$ 296,212	$ 1,025,277	$ 823,189
Operating income (loss)						34,725	10,290	27,618	(3,291)
Net loss after taxes						23,910	6,291	7,931	(5,506)
(Gains) for changes in fair values of contingent consideration						1,163	(6)	1,833	446
Purchase price allocation
Goodwill						578,945		578,945		$ 333,626	$ 333,626
Nuvotronics
Sales and results of operations
Sales						3,600		4,300
Operating income (loss)						(3,200)		(4,900)
Net loss after taxes						(3,200)		(4,900)
Amortization						600		700
Acquisition-related (benefit) expenses						1,100		$ 2,900
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred		$ 66,800
Cash consideration paid		61,500
Payable due to the sellers		400
Contingent consideration		4,900
Period for contingent consideration contracts								12 months
Purchase price allocation
Accounts receivable and contract assets		2,600
Fixed assets		2,700
Accounts payable and accrued expenses		(1,800)
Deferred taxes		(3,500)
Other net assets acquired (liabilities assumed)		600
Net identifiable assets acquired		26,100
Goodwill		40,700
Net assets acquired		$ 66,800
Weighted average useful life of intangible assets		9 years
Estimated amortization expense related to the intangible assets
2019											1,200
2020											4,000
2021											3,000
2022											3,000
2023											2,900
Thereafter											12,600
Nuvotronics | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration		$ 8,000				8,000		$ 8,000
Nuvotronics | Trade names
Purchase price allocation
Indefinite intangible assets		1,500
Nuvotronics | Technology
Purchase price allocation
Amortizable intangible assets		22,700
Nuvotronics | Customer relationships
Purchase price allocation
Amortizable intangible assets		600
Nuvotronics | Backlog
Purchase price allocation
Amortizable intangible assets		1,400
Nuvotronics | Non-compete agreements
Purchase price allocation
Amortizable intangible assets		$ 500
GRIDSMART
Sales and results of operations
Sales						7,700		14,000
Operating income (loss)						700		(1,300)
Net loss after taxes						700		(1,300)
Amortization						1,300		2,700
Acquisition-related (benefit) expenses						600		2,400
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred			$ 86,800
Cash consideration paid			87,200
Receivable due from the sellers			400
Purchase price allocation
Accounts receivable			1,700
Inventory			4,300
Accounts payable and accrued expenses			(2,500)
Deferred taxes			(3,900)
Other net assets acquired (liabilities assumed)			500
Net identifiable assets acquired			31,800
Goodwill			55,000
Net assets acquired			$ 86,800
Weighted average useful life of intangible assets			8 years
Estimated amortization expense related to the intangible assets
2019											4,000
2020											5,300
2021											3,900
2022											3,500
2023											3,500
Thereafter											11,500
GRIDSMART | Trade names
Purchase price allocation
Indefinite intangible assets			$ 2,400
GRIDSMART | Technology
Purchase price allocation
Amortizable intangible assets			25,700
GRIDSMART | Customer relationships
Purchase price allocation
Amortizable intangible assets			$ 3,600
Trafficware
Sales and results of operations
Sales						15,800		38,000
Operating income (loss)						(1,000)		(9,500)
Net loss after taxes						(1,000)		(9,500)
Amortization						2,800		12,400
Acquisition-related (benefit) expenses						900		4,400
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred				$ 237,200
Purchase price allocation
Accounts receivable				10,400
Inventory				9,900
Accounts payable and accrued expenses				(6,600)
Other net assets acquired (liabilities assumed)				1,900
Net identifiable assets acquired				86,400
Goodwill				150,800
Net assets acquired				$ 237,200
Weighted average useful life of intangible assets				7 years
Estimated amortization expense related to the intangible assets
2019											15,300
2020											11,400
2021											11,400
2022											11,400
2023											6,400
Thereafter											18,700
Trafficware | Trade names
Purchase price allocation
Indefinite intangible assets				$ 4,600
Trafficware | Technology
Purchase price allocation
Amortizable intangible assets				43,300
Trafficware | Customer relationships
Purchase price allocation
Amortizable intangible assets				21,900
Trafficware | Backlog
Purchase price allocation
Amortizable intangible assets				$ 4,800
Shield Aviation
Sales and results of operations
Operating income (loss)						(2,000)		(4,800)
Net loss after taxes						(2,000)		(4,800)
Amortization						200		600
(Gains) for changes in fair values of contingent consideration						(700)		(400)
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred					$ 12,800
Extinguishment of debt					5,200
Cash consideration paid					1,300
Hold back consideration					700
Contingent consideration					5,600
Purchase price allocation
Other net assets acquired					300
Net identifiable assets acquired					6,300
Goodwill					6,500
Net assets acquired					12,800
Estimated amortization expense related to the intangible assets
2019											800
2020											800
2021											800
2022											800
2023											800
Thereafter											2,100
Shield Aviation | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration					10,000	10,000		10,000
Shield Aviation | Technology
Purchase price allocation
Amortizable intangible assets					$ 6,000
Weighted average useful life of intangible assets					8 years
MotionDSP
Sales and results of operations
Sales						600	300	1,100	400
Operating income (loss)						(100)	(200)	(500)	(1,000)
Net loss after taxes						(100)	(200)	(500)	(1,000)
Amortization						$ 200	200	600	200
Acquisition-related (benefit) expenses							$ 100	$ 200	$ 700
Estimated acquisition-date fair value of consideration
Cash consideration paid	$ 4,800
Purchase price allocation
Accounts payable and accrued expenses	(300)
Other noncurrent liabilities	(800)
Other net assets acquired (liabilities assumed)	900
Net identifiable assets acquired	2,800
Goodwill	6,700
Net assets acquired	$ 9,500
Weighted average useful life of intangible assets	7 years
Estimated amortization expense related to the intangible assets
2019											700
2020											700
2021											700
2022											700
2023											700
Thereafter											$ 800
MotionDSP | Trade names
Purchase price allocation
Indefinite intangible assets	$ 100
MotionDSP | Technology
Purchase price allocation
Amortizable intangible assets	4,500
MotionDSP | Customer relationships
Purchase price allocation
Amortizable intangible assets	$ 200