Revenue - Disaggregation of Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jul. 01, 2018	Jun. 30, 2019	Jul. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 1,367.5	$ 1,386.9	$ 2,750.1	$ 2,756.8
Outcome-Based Services
Disaggregation of Revenue [Line Items]
Revenue	153.9	123.1	309.3	239.3
Global Talent Solutions
Disaggregation of Revenue [Line Items]
Revenue	505.9	500.7	1,006.9	986.5
Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	597.6	604.0	1,224.1	1,208.3
International Staffing
Disaggregation of Revenue [Line Items]
Revenue	268.1	286.6	527.0	571.3
Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	352.0	377.6	697.6	747.2
Commercial | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	384.1	414.5	776.6	812.9
Educational Staffing | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	117.4	110.0	257.0	240.1
Professional/Technical | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	87.3	70.1	172.8	137.5
Permanent Placement | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	8.8	9.4	17.7	17.8
Permanent Placement | International Staffing
Disaggregation of Revenue [Line Items]
Revenue	6.4	7.5	13.1	15.3
Permanent Placement | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	0.5	0.4	0.8	0.8
Staffing Solutions | International Staffing
Disaggregation of Revenue [Line Items]
Revenue	261.7	279.1	513.9	556.0
Staffing Solutions | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	258.2	288.3	513.8	572.5
Talent Solutions | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	93.3	88.9	183.0	173.9
Less: Intersegment
Disaggregation of Revenue [Line Items]
Revenue	$ (4.1)	$ (4.4)	$ (7.9)	$ (9.3)