Acquisitions and Divestitures - Narrative (Details) $ in Thousands, $ in Millions	3 Months Ended				6 Months Ended			12 Months Ended
	Sep. 27, 2020 USD ($)	Sep. 27, 2020 AUD ($)	Sep. 29, 2019 USD ($)	Jul. 01, 2018 USD ($)	Mar. 29, 2020 USD ($)	Mar. 31, 2019 USD ($)	Apr. 01, 2018 AUD ($)	Sep. 27, 2020 USD ($)	Sep. 29, 2019 USD ($) employee	Sep. 29, 2019 AUD ($) employee	Sep. 30, 2018 USD ($) employee	Sep. 30, 2018 AUD ($) employee	Sep. 27, 2020 AUD ($)
Business Acquisition
Contingent earn-out liability	$ 32,600							$ 32,600
Aggregate maximum of contingent consideration	70,900							70,900
Proceeds from divestiture of business								17,710	$ 651		$ 35,348
Fair value adjustments to contingent consideration liabilities								$ 14,971	(1,085)		(4,252)
Minimum
Business Acquisition
Earn-out period								2 years
Maximum
Business Acquisition
Earn-out period								3 years
Existing customer contracts | Minimum
Business Acquisition
Useful life of intangible assets								1 year
Existing customer contracts | Maximum
Business Acquisition
Useful life of intangible assets								10 years
Technology and trade names | Minimum
Business Acquisition
Useful life of intangible assets								3 years
Technology and trade names | Maximum
Business Acquisition
Useful life of intangible assets								5 years
Non-core field services business | Disposed of by Sale
Business Acquisition
Proceeds from divestiture of business											30,200
Disposal group revenue											$ 70,000
Loss on disposition of business				$ 3,400
Glumac
Business Acquisition
Number of employees | employee											300	300
Aggregate fair value of purchase prices											$ 38,400
Cash paid to the sellers											20,000
Contingent earn-out liability											18,400
Aggregate maximum of contingent consideration											$ 20,000
Earn-out period											3 years	3 years
NDY
Business Acquisition
Number of employees | employee											700	700
Aggregate fair value of purchase prices											$ 56,100
Cash paid to the sellers											46,900
Contingent earn-out liability	1,800						$ 9.4	$ 1,800			7,600		$ 2.6
Aggregate maximum of contingent consideration							$ 25.0				$ 20,200
Earn-out period							3 years				3 years	3 years
Escrow deposit											$ 1,600
Fair value adjustments to contingent consideration liabilities	3,700	$ 5.2							(5,400)	$ (7.9)	$ (2,100)	$ (3.0)
Maximum contingent consideration, year one							$ 7.4
Maximum contingent consideration, year two							8.8
Maximum contingent consideration, year three							$ 8.8
Percentage of maximum potential earn-out							38.00%
eGlobalTech
Business Acquisition
Aggregate fair value of purchase prices									49,100
Contingent earn-out liability	7,500		$ 21,100			$ 21,100		7,500	21,100
Aggregate maximum of contingent consideration			25,000			$ 25,000			$ 25,000
Earn-out period						3 years			3 years	3 years
Debt assumed									$ 24,700
Purchase price related to payables			3,300						$ 3,300
Fair value adjustments to contingent consideration liabilities	4,700
Maximum contingent consideration, year one						$ 8,500
Maximum contingent consideration, year two						9,000
Maximum contingent consideration, year three						$ 7,500
Percentage of maximum potential earn-out						84.00%
WYG plc
Business Acquisition
Number of employees | employee									1,600	1,600
Aggregate fair value of purchase prices									$ 54,200
Debt assumed									$ 11,500
Acquisition and integration costs			$ 10,400
SEG
Business Acquisition
Aggregate fair value of purchase prices								40,900
Cash paid to the sellers								29,600
Contingent earn-out liability	8,100				$ 11,300			8,100
Aggregate maximum of contingent consideration	20,000				$ 20,000			$ 20,000
Earn-out period					3 years			3 years
Fair value adjustments to contingent consideration liabilities	3,400
Maximum contingent consideration, year one					$ 5,000
Maximum contingent consideration, year two					7,000
Maximum contingent consideration, year three					$ 8,000
Percentage of maximum potential earn-out					57.00%
BWF
Business Acquisition
Aggregate fair value of purchase prices								$ 48,500
Cash paid to the sellers								41,800
Contingent earn-out liability	5,200							5,200
Aggregate maximum of contingent consideration	$ 8,000							$ 8,000
Earn-out period								3 years
Payables related to estimated post-closing adjustments								$ 1,500