Revenue - Disaggregation of Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Sep. 30, 2018	Oct. 01, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 1,342.4	$ 1,328.8	$ 4,099.2	$ 3,952.1
Outcome-Based Services
Disaggregation of Revenue [Line Items]
Revenue	134.0		373.3
Global Talent Solutions
Disaggregation of Revenue [Line Items]
Revenue	507.6		1,494.1
Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	561.8		1,770.1
International Staffing
Disaggregation of Revenue [Line Items]
Revenue	277.2		848.5
Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	373.6		1,120.8
Commercial | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	424.3		1,237.2
Educational Staffing | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	57.7		297.8
Professional/Technical | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	68.8		206.3
Permanent Placement | Americas Staffing
Disaggregation of Revenue [Line Items]
Revenue	11.0		28.8
Permanent Placement | International Staffing
Disaggregation of Revenue [Line Items]
Revenue	6.8		22.1
Permanent Placement | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	0.6		1.4
Staffing Solutions | International Staffing
Disaggregation of Revenue [Line Items]
Revenue	270.4		826.4
Staffing Solutions | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	279.0		851.5
Talent Solutions | Talent Fulfillment
Disaggregation of Revenue [Line Items]
Revenue	94.0		267.9
Less: Intersegment
Disaggregation of Revenue [Line Items]
Revenue	$ (4.2)		$ (13.5)