Business Segments Segment Reporting Information (Details) - USD ($) $ in Millions		3 Months Ended									12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	[1]	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues		$ 1,025.2	$ 1,002.7	$ 972.3	$ 923.7	$ 929.7	$ 906.4	$ 878.5		$ 685.2	$ 3,923.9	$ 3,399.8	$ 2,626.0
Gross profit		$ 289.5	$ 291.4	$ 285.2	$ 263.9	$ 272.0	$ 270.1	$ 263.9		$ 217.7	1,130.0	1,023.7	850.1
Operating Income (Loss)											308.6	260.2	224.7
Amortization of intangible assets											51.1	58.5	33.4
Apex
Segment Reporting Information [Line Items]
Revenues											2,520.0	2,300.3	2,037.2
Gross profit											746.0	687.9	606.3
Operating Income (Loss)											287.7	262.4	222.0
Amortization of intangible assets											20.3	26.2	29.4
Oxford
Segment Reporting Information [Line Items]
Revenues											605.7	606.5	588.8
Gross profit											242.9	248.9	243.8
Operating Income (Loss)											48.4	54.1	53.8
Amortization of intangible assets											3.7	4.2	4.0
ECS
Segment Reporting Information [Line Items]
Revenues											798.2	493.0	0.0
Gross profit											141.1	86.9	0.0
Operating Income (Loss)											42.2	15.5	0.0
Amortization of intangible assets											27.1	28.1	0.0
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual											798.2	493.0	0.0
Corporate
Segment Reporting Information [Line Items]
Operating Income (Loss)	[2]										$ (69.7)	$ (71.8)	$ (51.1)

[1] ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

[2] Corporate expenses primarily consist of consolidated stock-based compensation expense, compensation for corporate employees, acquisition, integration and strategic planning expenses, public company expenses and depreciation expense for corporate assets.