Revenue - Disaggregation of Revenues by Service Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Oct. 03, 2021	Sep. 27, 2020	Oct. 03, 2021	Sep. 27, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 1,195.4	$ 1,038.2	$ 3,659.4	$ 3,274.6
Professional & Industrial
Disaggregation of Revenue [Line Items]
Revenue	452.6	446.5	1,386.7	1,346.7
Science, Engineering & Technology
Disaggregation of Revenue [Line Items]
Revenue	306.2	244.0	859.1	761.5
Education
Disaggregation of Revenue [Line Items]
Revenue	66.6	27.5	284.1	195.1
Outsourcing & Consulting
Disaggregation of Revenue [Line Items]
Revenue	113.4	87.9	320.0	261.0
International
Disaggregation of Revenue [Line Items]
Revenue	256.8	232.4	810.1	710.6
Staffing Services | Professional & Industrial
Disaggregation of Revenue [Line Items]
Revenue	344.7	341.3	1,057.0	1,033.1
Staffing Services | Science, Engineering & Technology
Disaggregation of Revenue [Line Items]
Revenue	214.0	181.6	613.1	562.3
Staffing Services | Education
Disaggregation of Revenue [Line Items]
Revenue	65.7	27.4	280.6	194.9
Staffing Services | International
Disaggregation of Revenue [Line Items]
Revenue	247.1	228.8	784.7	697.9
Permanent Placement | Professional & Industrial
Disaggregation of Revenue [Line Items]
Revenue	6.9	2.3	17.1	7.0
Permanent Placement | Science, Engineering & Technology
Disaggregation of Revenue [Line Items]
Revenue	6.5	3.1	17.4	9.0
Permanent Placement | Education
Disaggregation of Revenue [Line Items]
Revenue	0.9	0.1	3.5	0.2
Permanent Placement | International
Disaggregation of Revenue [Line Items]
Revenue	5.4	3.6	16.3	12.7
Outcome-Based Services | Professional & Industrial
Disaggregation of Revenue [Line Items]
Revenue	101.0	102.9	312.6	306.6
Outcome-Based Services | Science, Engineering & Technology
Disaggregation of Revenue [Line Items]
Revenue	85.7	59.3	228.6	190.2
Talent Solutions | Outsourcing & Consulting
Disaggregation of Revenue [Line Items]
Revenue	113.4	87.9	320.0	261.0
Talent Solutions | International
Disaggregation of Revenue [Line Items]
Revenue	4.3	0.0	9.1	0.0
Less: Intersegment
Disaggregation of Revenue [Line Items]
Revenue	$ (0.2)	$ (0.1)	$ (0.6)	$ (0.3)