Acquisitions and Divestitures - Consolidated Business Acquisitions (Details) - USD ($) $ in Thousands		1 Months Ended				3 Months Ended		6 Months Ended
	Feb. 21, 2018	Mar. 31, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Oct. 01, 2018	Sep. 30, 2018
Sales and results of operations
Sales						$ 337,339	$ 278,586	$ 642,598	$ 526,977
Operating income (loss)						(6,541)	(1,679)	(7,107)	(13,581)
Net loss after taxes						(9,392)	(2,011)	(15,979)	(11,797)
Purchase price allocation
Goodwill		$ 579,648				579,648		579,648		$ 333,626	$ 333,626
Nuvotronics
Sales and results of operations
Sales						700		700
Operating income (loss)						(1,700)		(1,700)
Net loss after taxes						(1,700)		(1,700)
Amortization						100		100
Acquisition-related (benefit) expenses						1,800		1,800
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred		66,200
Cash consideration paid		61,500
Receivable due from the sellers		200				200		200
Contingent consideration		4,900				4,900		$ 4,900
Period for contingent consideration contracts								12 months
Purchase price allocation
Accounts receivable		3,000				3,000		$ 3,000
Fixed assets		2,700				2,700		2,700
Accounts payable and accrued expenses		(2,400)				(2,400)		(2,400)
Deferred taxes		(3,500)				(3,500)		(3,500)
Other net assets acquired (liabilities assumed)		600				600		600
Net identifiable assets acquired		26,400				26,400		26,400
Goodwill		39,800				39,800		39,800
Net assets acquired		66,200				66,200		$ 66,200
Weighted average useful life of intangible assets								9 years
Estimated amortization expense related to the intangible assets
2019											1,300
2020											4,100
2021											3,000
2022											3,000
2023											2,900
Thereafter											12,900
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				1,500		1,500
Nuvotronics | Technology
Purchase price allocation
Amortizable intangible assets		23,000				23,000		23,000
Nuvotronics | Customer relationships
Purchase price allocation
Amortizable intangible assets		600				600		600
Nuvotronics | Backlog
Purchase price allocation
Amortizable intangible assets		1,400				1,400		1,400
Nuvotronics | Non-compete agreements
Purchase price allocation
Amortizable intangible assets		700				700		700
GRIDSMART
Sales and results of operations
Sales						6,300		6,300
Operating income (loss)						(2,000)		(2,000)
Net loss after taxes						(2,000)		(2,000)
Amortization						1,400		1,400
Acquisition-related (benefit) expenses						1,800		1,800
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						11,700		22,200
Operating income (loss)						(5,200)		(8,500)
Net loss after taxes						(5,200)		(8,500)
Amortization						5,300		9,600
Acquisition-related (benefit) expenses						2,100		3,500
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred				$ 237,600
Cash consideration paid				239,200
Receivable due from the sellers				1,600
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				151,200
Net assets acquired				$ 237,600
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											18,800
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)						(1,900)		(2,800)
Net loss after taxes						(1,900)		(2,800)
Amortization						200		400
Acquisition-related (benefit) expenses						300		300
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		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						200	100	400	100
Operating income (loss)						(200)	(200)	(600)	(200)
Net loss after taxes						(200)	(200)	(600)	(200)
Amortization						200	100	400	100
Acquisition-related (benefit) expenses						100	$ 400	200	$ 600
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				700		700
2020		700				700		700
2021		700				700		700
2022		700				700		700
2023		700				700		700
Thereafter		$ 800				$ 800		$ 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