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Goodwill and Identifiable Assets Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 02, 2018
Goodwill [Line Items]
Amortization of intangible assets	$ 51.7	$ 51.1	$ 58.5
Goodwill [Roll Forward]
Gross goodwill, period start	1,486.9	1,421.1
Translation adjustment	2.0	(0.3)
Gross goodwill, period end	1,618.4	1,486.9	1,421.1
Apex
Goodwill [Roll Forward]
Gross goodwill, period start	(703.5)	662.1
Translation adjustment	(0.1)
Gross goodwill, period end	(743.7)	(703.5)	662.1
Oxford
Goodwill [Roll Forward]
Gross goodwill, period start	(230.5)	230.8
Translation adjustment	2.1	(0.3)
Gross goodwill, period end	(232.6)	(230.5)	230.8
ECS
Goodwill [Roll Forward]
Gross goodwill, period start	552.9	528.2
Gross goodwill, period end	642.1	552.9	$ 528.2
Business Acquisition, Goodwill, Expected Tax Deductible Amount				$ 514.2
Business Acquisition, Goodwill, Expected Tax Deductible Amount				$ 514.2
2019 Acquisitions
Goodwill [Roll Forward]
Goodwill, Acquired During Period		66.1
Business Acquisition, Goodwill, Expected Tax Deductible Amount		63.3
Business Acquisition, Goodwill, Expected Tax Deductible Amount		63.3
2019 Acquisitions | Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period		41.4
2019 Acquisitions | ECS
Goodwill [Roll Forward]
Goodwill, Acquired During Period		$ 24.7
Other 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
Other Acquisitions | Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period	40.3
Other Acquisitions | ECS
Goodwill [Roll Forward]
Goodwill, Acquired During Period	$ 89.2

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- Definition

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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