Revenue - Disaggregation of Revenues by Geographic Region (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 27, 2020	Sep. 29, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 1,038.2	$ 1,267.7	$ 3,274.6	$ 4,017.8
Americas
Disaggregation of Revenue [Line Items]
Revenue	818.5	997.4	2,609.6	3,186.5
Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	7.6	7.5	21.1	19.6
Science, Engineering & Technology
Disaggregation of Revenue [Line Items]
Revenue	244.0	285.2	761.5	859.7
Science, Engineering & Technology | Americas
Disaggregation of Revenue [Line Items]
Revenue	242.8	283.2	757.3	852.3
Science, Engineering & Technology | Europe
Disaggregation of Revenue [Line Items]
Revenue	1.2	2.0	4.2	7.4
Outsourcing & Consulting
Disaggregation of Revenue [Line Items]
Revenue	87.9	94.4	261.0	282.3
Outsourcing & Consulting | Americas
Disaggregation of Revenue [Line Items]
Revenue	72.9	78.9	215.9	238.2
Outsourcing & Consulting | Europe
Disaggregation of Revenue [Line Items]
Revenue	7.4	8.0	24.0	24.5
Outsourcing & Consulting | Asia Pacific
Disaggregation of Revenue [Line Items]
Revenue	7.6	7.5	21.1	19.6
International
Disaggregation of Revenue [Line Items]
Revenue	232.4	293.4	710.6	893.6
International | Americas
Disaggregation of Revenue [Line Items]
Revenue	28.9	40.5	94.9	113.8
International | Europe
Disaggregation of Revenue [Line Items]
Revenue	$ 203.5	$ 252.9	$ 615.7	$ 779.8