Acquisitions and Divestitures - Consolidated Business Acquisitions (Details) - USD ($) $ in Thousands					1 Months Ended			3 Months Ended								12 Months Ended
	Feb. 21, 2018	Nov. 30, 2016	Feb. 02, 2016	Dec. 21, 2015	Oct. 31, 2018	Jul. 31, 2018	Jul. 31, 2017	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Oct. 31, 2017
Sales and results of operations
Sales								$ 379,709	$ 296,212	$ 278,586	$ 248,391	$ 349,115	$ 266,184	$ 248,040	$ 244,370	$ 1,202,898	$ 1,107,709	$ 1,070,601
Operating income (loss)								27,673	10,290	(1,679)	(11,902)	21,207	(6,872)	(6,084)	(5,623)	24,382	2,628	(11,649)
Net income (loss) after taxes								17,816	$ 6,291	$ (2,011)	$ (9,786)	13,155	$ (21,957)	$ 461	$ (2,868)	12,310	(11,209)	1,735
Estimated acquisition-date fair value of consideration
Compensation expense																7,515	5,012	7,748
Compensation expense not expected to be deductible for tax purposes																		6,300
Purchase price allocation
Goodwill								333,626				$ 321,562				333,626	321,562	312,596
Estimated amortization expense related to the intangible assets
2019								20,162								20,162
2020								15,885								15,885
2021								12,211								12,211
2022								8,981								8,981
2023								6,795								6,795
Thereafter								9,499								9,499
Shield Aviation
Sales and results of operations
Operating income (loss)																(800)
Net income (loss) after taxes																(600)
Amortization																100
Acquisition-related (benefit) expenses																200
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
Weighted average useful life of intangible assets						8 years
Estimated amortization expense related to the intangible assets
2019								800								800
2020								800								800
2021								800								800
2022								800								800
2023								800								800
Thereafter								1,900								1,900
Shield Aviation | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration						$ 10,000
Shield Aviation | Technology
Purchase price allocation
Amortizable intangible assets						$ 6,000
MotionDSP
Sales and results of operations
Sales																600
Operating income (loss)																(2,700)
Net income (loss) after taxes																(1,900)
Amortization																400
Acquisition-related (benefit) expenses																800	200
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
2020								700								700
2021								700								700
2022								700								700
2023								700								700
Thereafter								900								900
MotionDSP | Trade names
Purchase price allocation
Indefinite intangible assets	$ 100
MotionDSP | Customer relationships
Purchase price allocation
Amortizable intangible assets	200
MotionDSP | Technology
Purchase price allocation
Amortizable intangible assets	$ 4,500
Deltenna
Sales and results of operations
Sales																300	100
Operating income (loss)																200	(200)
Net income (loss) after taxes																200	(200)
Amortization																300
Gains (losses) for changes in fair value of contingent consideration																(300)
Acquisition-related (benefit) expenses																	200
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred							$ 5,300
Cash consideration paid							4,000
Contingent consideration							1,300
Purchase price allocation
Other noncurrent liabilities							(300)
Net identifiable assets acquired							1,800
Goodwill							3,500
Net assets acquired							$ 5,300
Weighted average useful life of intangible assets							8 years
Estimated amortization expense related to the intangible assets
2019								300								300
2020								300								300
2021								300								300
2022								300								300
2023								300								300
Thereafter								300								300
Deltenna | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration							$ 7,000
Deltenna | Customer relationships
Purchase price allocation
Amortizable intangible assets							1,000
Deltenna | Technology
Purchase price allocation
Amortizable intangible assets							$ 1,100
Vocality
Sales and results of operations
Sales																5,100	1,500
Operating income (loss)																(1,300)	(2,900)
Net income (loss) after taxes																(1,200)	(2,600)
Amortization																800	600
Acquisition-related (benefit) expenses																600	1,600
Estimated acquisition-date fair value of consideration
Compensation expense																	400
Fair value of consideration transferred		$ 9,600
Cash consideration paid		9,700
Contingent consideration		300
Cash paid related to compensation expenses		400
Purchase price allocation
Inventory		1,700
Accounts payable and accrued expenses		(400)
Other noncurrent liabilities		(500)
Net identifiable assets acquired		5,700
Goodwill		3,900
Net assets acquired		$ 9,600
Weighted average useful life of intangible assets		9 years
Estimated amortization expense related to the intangible assets
2019								700								700
2020								600								600
2021								500								500
2022								500								500
2023								400								400
Thereafter								900								900
Vocality | Trade names
Purchase price allocation
Indefinite intangible assets		$ 400
Vocality | Customer relationships
Purchase price allocation
Amortizable intangible assets		2,100
Vocality | Technology
Purchase price allocation
Amortizable intangible assets		$ 2,400
GATR
Sales and results of operations
Sales																111,100	84,300	43,100
Operating income (loss)																4,000	1,900	(26,400)
Net income (loss) after taxes																2,900	1,400	(23,000)
Amortization																11,100	12,700	9,700
Gains (losses) for changes in fair value of contingent consideration																	(3,200)	700
Acquisition-related (benefit) expenses																400	600	22,000
Estimated acquisition-date fair value of consideration
Compensation expense																		18,500
Compensation expense not expected to be deductible for tax purposes																		15,400
Fair value of consideration transferred			$ 220,500
Cash consideration paid			236,100
Contingent consideration			2,500
Cash paid related to compensation expenses			18,100
Purchase price allocation
Accounts receivable			10,600
Inventory			3,400
Income tax receivable			5,100
Deferred tax liabilities			23,800
Accounts payable and accrued expenses			(2,400)
Net identifiable assets acquired			64,600
Goodwill			155,900
Net assets acquired			$ 220,500
Weighted average useful life of intangible assets			9 years
Estimated amortization expense related to the intangible assets
2019								9,800								9,800
2020								8,300								8,300
2021								6,900								6,900
2022								5,600								5,600
2023								3,800								3,800
Thereafter								3,800								3,800
GATR | Trade names
Purchase price allocation
Indefinite intangible assets			$ 4,700
GATR | Customer relationships
Purchase price allocation
Amortizable intangible assets			51,700
GATR | Backlog
Purchase price allocation
Amortizable intangible assets			3,400
GATR | Technology
Purchase price allocation
Amortizable intangible assets			10,700
GATR | Non-compete agreements
Purchase price allocation
Amortizable intangible assets			$ 1,200
TeraLogics
Sales and results of operations
Sales																23,700	19,700	14,200
Operating income (loss)																300	(1,800)	(2,900)
Net income (loss) after taxes																200	(1,200)	(1,600)
Amortization																2,800	3,500	3,000
Gains (losses) for changes in fair value of contingent consideration																1,300	1,300	1,500
Acquisition-related (benefit) expenses																	$ 200	2,300
Estimated acquisition-date fair value of consideration
Compensation expense																		$ 1,300
Fair value of consideration transferred				$ 33,900
Cash consideration paid				28,900
Contingent consideration				5,000
Purchase price allocation
Accounts receivable				1,400
Accounts payable and accrued expenses				(500)
Other net assets acquired (liabilities assumed)				(100)
Net identifiable assets acquired				15,700
Goodwill				18,200
Net assets acquired				$ 33,900
Weighted average useful life of intangible assets				7 years
Estimated amortization expense related to the intangible assets
2019								2,100								2,100
2020								1,400								1,400
2021								800								800
2022								500								500
2023								400								400
Thereafter								$ 500								$ 500
TeraLogics | Customer relationships
Purchase price allocation
Amortizable intangible assets				$ 6,700
TeraLogics | Backlog
Purchase price allocation
Amortizable intangible assets				5,600
TeraLogics | Software.
Purchase price allocation
Amortizable intangible assets				2,500
TeraLogics | Non-compete agreements
Purchase price allocation
Amortizable intangible assets				$ 100
Trafficware | Subsequent event
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred					$ 235,700
MotionDSP
Acquisitions
Purchase price of capital stock																			$ 4,700
Ownership percentage																			49.00%