Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues:
Revenues		$ 1,011.9	$ 1,002.7	$ 2,939.2	$ 2,898.7
Gross Profit:
Total Gross Profit		271.0	291.4	812.7	840.5
Operating Income (Loss):
Total Operating Income	[1]	80.9	90.9	227.8	226.9
Amortization of intangible assets		12.9	11.9	37.6	38.8
Apex
Revenues:
Revenues		596.1	644.1	1,802.1	1,878.7
Gross Profit:
Total Gross Profit		173.0	192.2	528.1	555.4
Operating Income (Loss):
Total Operating Income		69.4	79.2	202.3	213.1
Amortization of intangible assets		5.7	4.0	16.5	15.5
Oxford
Revenues:
Revenues		127.2	152.5	396.1	455.3
Gross Profit:
Total Gross Profit		52.1	61.9	157.8	183.3
Operating Income (Loss):
Total Operating Income		12.1	13.6	31.0	36.3
Amortization of intangible assets		0.2	1.0	0.5	3.0
ECS
Revenues:
Revenues		288.6	206.1	741.0	564.7
Gross Profit:
Total Gross Profit		45.9	37.3	126.8	101.8
Operating Income (Loss):
Total Operating Income		16.7	12.7	44.3	30.5
Amortization of intangible assets		7.0	6.9	20.6	20.3
Assignment | Apex
Revenues:
Revenues		492.2	544.7	1,502.3	1,598.0
Assignment | Oxford
Revenues:
Revenues		95.8	115.3	306.9	350.0
Consulting | Apex
Revenues:
Revenues		96.6	85.1	271.5	237.0
Consulting | Oxford
Revenues:
Revenues		14.1	15.0	38.4	41.1
Permanent Placement | Apex
Revenues:
Revenues		7.3	14.3	28.3	43.7
Permanent Placement | Oxford
Revenues:
Revenues		17.3	22.2	50.8	64.2
Firm-fixed-price | ECS
Revenues:
Revenues		82.0	64.4	201.2	153.8
Time and materials | ECS
Revenues:
Revenues		80.8	68.4	234.6	200.6
Cost reimbursable | ECS
Revenues:
Revenues		$ 125.8	$ 73.3	$ 305.2	$ 210.3

[1] ___________________


(1) Consolidated operating income includes corporate operating expenses, which are not allocated to the segments, consisting 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.