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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 |  |  |  |  |  |

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