Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues:
Revenues		$ 972.3	$ 878.5	$ 1,896.0	$ 1,563.7
Gross Profit:
Total Gross Profit		285.2	263.9	549.1	481.6
Operating Income (Loss):
Total Operating Income		73.3	65.7	136.0	111.4
Amortization of Intangible Assets		13.1	18.5	26.9	26.1
Apex
Revenues:
Revenues		628.5	567.6	1,234.6	1,106.1
Gross Profit:
Total Gross Profit		187.8	169.7	363.2	328.3
Operating Income (Loss):
Total Operating Income		72.9	64.9	133.9	121.2
Amortization of Intangible Assets		5.5	6.6	11.5	13.1
Oxford
Revenues:
Revenues		153.2	155.8	302.8	302.5
Gross Profit:
Total Gross Profit		62.5	65.5	121.4	124.6
Operating Income (Loss):
Total Operating Income		11.0	14.8	22.7	24.6
Amortization of Intangible Assets		1.0	1.0	2.0	2.1
ECS
Revenues:
Revenues		190.6	155.1	358.6	155.1
Gross Profit:
Total Gross Profit		34.9	28.7	64.5	28.7
Operating Income (Loss):
Total Operating Income		10.9	3.7	17.8	3.7
Amortization of Intangible Assets		6.6	10.9	13.4	10.9
Corporate
Operating Income (Loss):
Total Operating Income	[1]	(21.5)	(17.7)	(38.4)	(38.1)
Assignment | Apex
Revenues:
Revenues		613.0	553.7	1,205.2	1,078.6
Assignment | Oxford
Revenues:
Revenues		131.4	130.3	260.8	255.7
Permanent Placement | Apex
Revenues:
Revenues		15.5	13.9	29.4	27.5
Permanent Placement | Oxford
Revenues:
Revenues		21.8	25.5	42.0	46.8
Fixed-price Contract | ECS
Revenues:
Revenues		46.1	44.8	89.4	44.8
Time-and-materials Contract | ECS
Revenues:
Revenues		70.8	49.0	132.2	49.0
Cost-plus-fixed-fee Contract | ECS
Revenues:
Revenues		$ 73.7	$ 61.3	$ 137.0	$ 61.3

[1] 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.