Revenue - Disaggregation of Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 975.3	$ 1,367.5	$ 2,236.4	$ 2,750.1
Outcome-Based Services
Disaggregation of Revenue [Line Items]
Revenue	169.8	153.9	342.0	309.3
Global Talent Solutions
Disaggregation of Revenue [Line Items]
Revenue	466.9	505.9	970.1	1,006.9
Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	326.7	597.6	860.1	1,224.1
International Staffing
Disaggregation of Revenue [Line Items]
Revenue	184.6	268.1	412.2	527.0
Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	297.1	352.0	628.1	697.6
Commercial | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	233.6	384.1	542.1	776.6
Education | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	25.2	117.4	168.3	257.0
Professional/Technical | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	63.8	87.3	139.2	172.8
Permanent Placement | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	4.1	8.8	10.5	17.7
Permanent Placement | International Staffing
Disaggregation of Revenue [Line Items]
Revenue	3.2	6.4	8.9	13.1
Permanent Placement | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	0.3	0.5	0.6	0.8
Staffing Solutions | International Staffing
Disaggregation of Revenue [Line Items]
Revenue	181.4	261.7	403.3	513.9
Staffing Solutions | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	216.5	258.2	461.0	513.8
Talent Solutions | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	80.3	93.3	166.5	183.0
Less: Intersegment
Disaggregation of Revenue [Line Items]
Revenue	$ (2.9)	$ (4.1)	$ (6.0)	$ (7.9)