Acquisitions and Divestitures - Consolidated Business Acquisitions (Details) - USD ($) $ in Thousands		1 Months Ended				3 Months Ended								6 Months Ended	12 Months Ended			15 Months Ended
	Jan. 03, 2020	Mar. 31, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jan. 03, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Jan. 03, 2020
Purchase price allocation
Goodwill						$ 784,882				$ 578,097					$ 784,882	$ 578,097	$ 333,626
Sales and results of operations
Sales						475,475	$ 350,439	$ 321,482	$ 328,839	471,198	$ 382,679	$ 337,339	$ 305,259		1,476,235	1,496,475	1,202,898
Operating income (loss)						73,286	24,706	(29,934)	(6,483)	58,619	34,725	(6,541)	(566)		61,575	86,237	24,382
Net income (loss) after taxes						57,632	$ (1,155)	$ (39,123)	$ (20,575)	$ 41,763	$ 23,910	(9,392)	$ (6,587)		(3,221)	49,694	12,310
(Gain) loss for changes in fair values of contingent consideration															(1,856)	(1,005)	$ 1,029
Estimated amortization expense related to the intangible assets
2021						60,728									60,728
2022						41,055									41,055
2023						28,468									28,468
2024						25,939									25,939
2025						24,375									24,375
Thereafter						29,796									$ 29,796
Delerrok Inc.
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred	$ 45,100
Cash consideration paid	37,000																	$ 43,500
Contingent consideration	1,600													$ 1,600				1,600
Period for contingent consideration contracts															12 months
Purchase price allocation
Other net assets acquired	200													200				200
Accounts receivable	900													900				900
Deferred tax liability	(2,000)													(2,000)				(2,000)
Net identifiable assets acquired	14,500													14,500				14,500
Goodwill	30,600													30,600				30,600
Net assets acquired	$ 45,100													45,100				45,100
Weighted average useful life of intangible assets	10 years
Sales and results of operations
Sales															$ 1,700
Operating income (loss)															(2,400)
Net income (loss) after taxes															(2,400)
Amortization															1,300
Acquisition-related expenses															1,600
(Gain) loss for changes in fair values of contingent consideration															(700)
Estimated amortization expense related to the intangible assets
2021						1,700									1,700
2022						1,600									1,600
2023						1,600									1,600
2024						1,600									1,600
2025						1,600									1,600
Thereafter						6,400									6,400
Delerrok Inc. | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration for certain goals															2,000
Delerrok Inc. | Trade names
Purchase price allocation
Indefinite intangible assets	$ 900													900				900
Delerrok Inc. | Technology
Purchase price allocation
Amortizable intangible assets	14,900													14,900				14,900
GRIDSMART
Purchase price allocation
Accounts receivable			$ 1,700
Inventory			4,300
Accounts payable and accrued expenses			(1,900)
Deferred tax liability			(3,300)
Other net assets acquired (liabilities assumed)			(500)
Net identifiable assets acquired			33,000
Goodwill			53,800
Net assets acquired			$ 86,800
Weighted average useful life of intangible assets			8 years
Sales and results of operations
Sales															27,800	20,600
Operating income (loss)															3,600	900
Net income (loss) after taxes															3,600	900
Amortization															5,300	4,000
Acquisition-related expenses															700	2,900
Estimated amortization expense related to the intangible assets
2021						3,900									3,900
2022						3,500									3,500
2023						3,500									3,500
2024						3,500									3,500
2025						3,500									3,500
Thereafter						4,600									4,600
GRIDSMART | Trade names
Purchase price allocation
Indefinite intangible assets			$ 2,400
GRIDSMART | Customer relationships
Purchase price allocation
Amortizable intangible assets			3,600
GRIDSMART | Technology
Purchase price allocation
Amortizable intangible assets			$ 25,700
Trafficware
Purchase price allocation
Accounts receivable				$ 10,400
Inventory				9,900
Accounts payable and accrued expenses				(9,500)
Other net assets acquired (liabilities assumed)				(2,000)
Net identifiable assets acquired				83,400
Goodwill				153,800
Net assets acquired				$ 237,200
Weighted average useful life of intangible assets				7 years
Sales and results of operations
Sales															56,000	53,800
Operating income (loss)															(3,600)	(11,000)
Net income (loss) after taxes															(3,600)	(11,000)
Amortization															11,400	15,300
Acquisition-related expenses															1,100	5,200
Estimated amortization expense related to the intangible assets
2021						11,400									11,400
2022						11,400									11,400
2023						6,400									6,400
2024						5,900									5,900
2025						5,900									5,900
Thereafter						6,900									6,900
Trafficware | Trade names
Purchase price allocation
Indefinite intangible assets				$ 4,600
Trafficware | Backlog
Purchase price allocation
Amortizable intangible assets				4,800
Trafficware | Customer relationships
Purchase price allocation
Amortizable intangible assets				21,900
Trafficware | Technology
Purchase price allocation
Amortizable intangible assets				$ 43,300
PIXIA Corp
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred	197,800													245,200
Cash consideration paid														247,800
Purchase price allocation
Accounts receivable, prepaids and other assets	3,800													3,800				3,800
Deferred tax liability	(17,600)													(17,600)				(17,600)
Other net assets acquired (liabilities assumed)	(1,800)													(1,800)				(1,800)
Net identifiable assets acquired	72,200													72,200				72,200
Goodwill	173,000													173,000				173,000
Net assets acquired	$ 245,200													245,200				245,200
Weighted average useful life of intangible assets	4 years
Sales and results of operations
Sales															23,400
Operating income (loss)															(9,200)
Net income (loss) after taxes															(9,200)
Amortization															21,500
Acquisition-related expenses															4,200
Estimated amortization expense related to the intangible assets
2021						28,700									28,700
2022						12,700									12,700
2023						7,300									7,300
2024						7,300									7,300
2025						7,300									7,300
Thereafter						3,200									$ 3,200
PIXIA Corp | Trade names
Purchase price allocation
Indefinite intangible assets	$ 5,700													5,700				5,700
PIXIA Corp | Backlog
Purchase price allocation
Amortizable intangible assets	42,500													42,500				42,500
PIXIA Corp | Customer relationships
Purchase price allocation
Amortizable intangible assets	25,500													25,500				25,500
PIXIA Corp | Technology
Purchase price allocation
Amortizable intangible assets	$ 14,100													$ 14,100				$ 14,100
Nuvotronics
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred		$ 66,800
Period for contingent consideration contracts															12 months
Purchase price allocation
Accounts receivable and contract assets		2,600										2,600
Fixed assets		2,700										2,700
Accounts payable and accrued expenses		(1,800)										(1,800)
Deferred tax liability		(3,200)										(3,200)
Other net assets acquired (liabilities assumed)		(600)										(600)
Net identifiable assets acquired		26,400										26,400
Goodwill		40,400										40,400
Net assets acquired		$ 66,800										66,800
Weighted average useful life of intangible assets		9 years
Sales and results of operations
Sales															$ 13,500	7,400
Operating income (loss)															(9,800)	(6,900)
Net income (loss) after taxes															(9,800)	(6,900)
Amortization															4,000	1,200
Acquisition-related expenses															1,700	3,700
(Gain) loss for changes in fair values of contingent consideration															(4,200)	(700)
Estimated amortization expense related to the intangible assets
2021						3,000									3,000
2022						3,000									3,000
2023						2,900									2,900
2024						2,700									2,700
2025						2,500									2,500
Thereafter						7,600									7,600
Nuvotronics | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration						8,000									8,000
Nuvotronics | Trade names
Purchase price allocation
Indefinite intangible assets		$ 1,500										1,500
Nuvotronics | Backlog
Purchase price allocation
Amortizable intangible assets		2,000										2,000
Nuvotronics | Technology
Purchase price allocation
Amortizable intangible assets		22,700										22,700
Nuvotronics | Non-compete agreements
Purchase price allocation
Amortizable intangible assets		$ 500										$ 500
Shield Aviation
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred					$ 12,800
Purchase price allocation
Other net assets acquired (liabilities assumed)					(300)
Net identifiable assets acquired					6,300
Goodwill					6,500
Net assets acquired					$ 12,800
Weighted average useful life of intangible assets					8 years
Sales and results of operations
Operating income (loss)															(6,500)	(5,300)
Net income (loss) after taxes															(6,500)	(5,300)
Amortization															800	800
(Gain) loss for changes in fair values of contingent consideration															1,800	$ (1,800)
Estimated amortization expense related to the intangible assets
2021						800									800
2022						800									800
2023						800									800
2024						800									800
2025						800									800
Thereafter						600									600
Shield Aviation | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration						$ 10,000									$ 10,000
Shield Aviation | Technology
Purchase price allocation
Amortizable intangible assets					$ 6,000