Goodwill and Identifiable Assets (Goodwill) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Gross goodwill	$ 1,617.5	$ 1,618.4	$ 1,486.9
Goodwill, Purchase Accounting Adjustments	0.2
Translation adjustment	(1.1)	2.0
Apex
Goodwill [Roll Forward]
Gross goodwill	743.6	743.7	703.5
Translation adjustment	(0.1)	(0.1)
Oxford
Goodwill [Roll Forward]
Gross goodwill	231.6	232.6	230.5
Translation adjustment	(1.0)	2.1
ECS
Goodwill [Roll Forward]
Gross goodwill	642.3	642.1	$ 552.9
Goodwill, Purchase Accounting Adjustments	$ 0.2
2020 Acquisitions
Goodwill [Roll Forward]
Goodwill, Acquired During Period		129.5
Business Acquisition, Goodwill, Expected Tax Deductible Amount		77.1
Business Acquisition, Goodwill, Expected Tax Deductible Amount		77.1
2020 Acquisitions | Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period		40.3
2020 Acquisitions | ECS
Goodwill [Roll Forward]
Goodwill, Acquired During Period		$ 89.2