Segment Disclosures - Segment Revenue From Service (Details) - Service - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Oct. 03, 2021	Sep. 27, 2020	Oct. 03, 2021	Sep. 27, 2020
Revenue from Services
Segment revenue from services	$ 1,195.4	$ 1,038.2	$ 3,659.4	$ 3,274.6
Less: Intersegment revenue
Revenue from Services
Segment revenue from services	(0.2)	(0.1)	(0.6)	(0.3)
Professional & Industrial | Reporting Segments
Revenue from Services
Segment revenue from services	452.6	446.5	1,386.7	1,346.7
Science, Engineering & Technology | Reporting Segments
Revenue from Services
Segment revenue from services	306.2	244.0	859.1	761.5
Education | Reporting Segments
Revenue from Services
Segment revenue from services	66.6	27.5	284.1	195.1
Outsourcing & Consulting | Reporting Segments
Revenue from Services
Segment revenue from services	113.4	87.9	320.0	261.0
International | Reporting Segments
Revenue from Services
Segment revenue from services	$ 256.8	$ 232.4	$ 810.1	$ 710.6