Acquisitions - Narrative (Details) $ in Thousands, $ in Millions	3 Months Ended		6 Months Ended			12 Months Ended
	Sep. 27, 2020 USD ($)	Sep. 27, 2020 AUD ($)	Mar. 29, 2020 USD ($)	Mar. 31, 2019 USD ($)	Apr. 01, 2018 AUD ($)	Oct. 03, 2021 USD ($) employee	Sep. 27, 2020 USD ($) acquisition	Sep. 29, 2019 USD ($) employee acquisition	Sep. 29, 2019 AUD ($) employee acquisition	Sep. 30, 2018 USD ($)	Sep. 30, 2018 AUD ($)	Sep. 27, 2020 AUD ($)
Business Acquisition [Line Items]
Contingent earn-out liability						$ 59,300
Acquisition and integration costs								$ 10,400
Fair value adjustments to contingent consideration liabilities						$ 3,273	$ 14,971	(1,085)
Minimum
Business Acquisition [Line Items]
Earn-out period						3 years
Significant unobservable input, earn-out period						2 years
Maximum
Business Acquisition [Line Items]
Earn-out period						5 years
Significant unobservable input, earn-out period						3 years
Existing customer contracts | Minimum
Business Acquisition [Line Items]
Useful life of intangible assets						1 year
Existing customer contracts | Maximum
Business Acquisition [Line Items]
Useful life of intangible assets						10 years
Technology and trade names | Minimum
Business Acquisition [Line Items]
Useful life of intangible assets						3 years
Technology and trade names | Maximum
Business Acquisition [Line Items]
Useful life of intangible assets						5 years
Hoare Lea
Business Acquisition [Line Items]
Number of employees | employee						900
Business acquisitions
Business Acquisition [Line Items]
Fair value of acquisition purchase price						$ 151,700	88,600	103,300
Initial cash payments						101,400	71,400	54,200
Contingent earn-out liability	$ 16,500					50,300	16,500	21,100
Aggregate maximum of contingent consideration	28,000					$ 74,000	$ 28,000	$ 25,000
Earn-out period							3 years	3 years	3 years
Number of acquisitions | acquisition							2	2	2
Payables related to estimated post-closing adjustments							$ 700	$ 3,300
Promissory note								24,700
Net debt assumed								$ 11,500
Business acquisitions | Minimum
Business Acquisition [Line Items]
Earn-out period						3 years
Business acquisitions | Maximum
Business Acquisition [Line Items]
Earn-out period						4 years
WYG
Business Acquisition [Line Items]
Number of employees | employee								1,600	1,600
NDY
Business Acquisition [Line Items]
Contingent earn-out liability	1,800				$ 9.4		1,800					$ 2.6
Aggregate maximum of contingent consideration					$ 25.0
Earn-out period					3 years
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%
EGT
Business Acquisition [Line Items]
Contingent earn-out liability	7,500			$ 21,100			7,500
Aggregate maximum of contingent consideration				$ 25,000
Earn-out period				3 years
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%
SEG
Business Acquisition [Line Items]
Contingent earn-out liability	8,100		$ 11,300				$ 8,100
Aggregate maximum of contingent consideration			$ 20,000
Earn-out period			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%
All acquisitions
Business Acquisition [Line Items]
Aggregate maximum of contingent consideration						$ 105,400