Revenue - Disaggregation of Revenues by Geographic Region (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 29, 2024	Dec. 31, 2023	Jan. 01, 2023
Disaggregation of Revenue [Line Items]
Revenue from services	$ 4,331.8	$ 4,835.7	$ 4,965.4
Americas
Disaggregation of Revenue [Line Items]
Revenue from services	4,234.2	3,928.3	3,998.6
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue from services	55.7	43.7	42.1
Science, Engineering & Technology
Disaggregation of Revenue [Line Items]
Revenue from services	1,422.8	1,190.8	1,265.4
Science, Engineering & Technology | Americas
Disaggregation of Revenue [Line Items]
Revenue from services	1,412.1	1,175.2	1,250.3
Science, Engineering & Technology | Europe
Disaggregation of Revenue [Line Items]
Revenue from services	10.7	15.6	15.1
Outsourcing & Consulting
Disaggregation of Revenue [Line Items]
Revenue from services	468.3	454.7	468.0
Outsourcing & Consulting | Americas
Disaggregation of Revenue [Line Items]
Revenue from services	381.4	375.0	403.3
Outsourcing & Consulting | Europe
Disaggregation of Revenue [Line Items]
Revenue from services	31.2	36.0	22.6
Outsourcing & Consulting | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue from services	55.7	43.7	42.1
International
Disaggregation of Revenue [Line Items]
Revenue from services	0.0	812.1	887.0
International | Europe
Disaggregation of Revenue [Line Items]
Revenue from services	$ 0.0	$ 812.1	$ 887.0