Business Segments Segment Reporting Information (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 3,950.6	$ 3,923.9	$ 3,399.8
Gross profit	1,089.3	1,130.0	1,023.7
Operating Income (Loss)	310.4	308.6	260.2
Amortization of intangible assets	51.7	51.1	58.5
Apex
Segment Reporting Information [Line Items]
Revenues	2,421.2	2,520.0	2,300.3
Gross profit	710.8	746.0	687.9
Operating Income (Loss)	276.6	287.7	262.4
Amortization	22.9	20.3	26.2
Oxford
Segment Reporting Information [Line Items]
Revenues	525.2	605.7	606.5
Gross profit	209.6	242.9	248.9
Operating Income (Loss)	43.9	48.4	54.1
Amortization	0.7	3.7	4.2
ECS
Segment Reporting Information [Line Items]
Revenues	1,004.2	798.2	493.0
Gross profit	168.9	141.1	86.9
Operating Income (Loss)	58.0	42.2	15.5
Amortization	$ 28.1	$ 27.1	$ 28.1

The aggregate amount of recurring noncash expense charged against earnings in the period to allocate the cost of assets over their estimated remaining economic lives.

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