Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues:
Revenues	$ 936.8	$ 972.3	$ 1,927.3	$ 1,896.0
Gross Profit:
Total Gross Profit	260.8	285.2	541.7	549.1
Operating Income (Loss):
Total Operating Income	76.0	73.3	146.9	136.0
Amortization of Intangible Assets	12.6	13.1	24.7	26.9
Apex
Revenues:
Revenues	576.9	628.5	1,206.0	1,234.6
Gross Profit:
Total Gross Profit	170.6	187.8	355.1	363.2
Operating Income (Loss):
Total Operating Income	66.0	72.9	133.1	133.9
Amortization of Intangible Assets	5.5	5.5	10.8	11.5
Oxford
Revenues:
Revenues	120.2	153.2	268.9	302.8
Gross Profit:
Total Gross Profit	46.4	62.5	105.7	121.4
Operating Income (Loss):
Total Operating Income	7.8	11.0	18.9	22.7
Amortization of Intangible Assets	0.1	1.0	0.3	2.0
ECS
Revenues:
Revenues	239.7	190.6	452.4	358.6
Gross Profit:
Total Gross Profit	43.8	34.9	80.9	64.5
Operating Income (Loss):
Total Operating Income	16.2	10.9	27.5	17.8
Amortization of Intangible Assets	7.0	6.6	13.6	13.4
Assignment | Apex
Revenues:
Revenues	483.8	533.5	1,010.1	1,053.3
Assignment | Oxford
Revenues:
Revenues	96.2	117.1	211.1	234.7
IT consulting | Apex
Revenues:
Revenues	85.3	79.5	174.9	151.9
IT consulting | Oxford
Revenues:
Revenues	11.1	14.3	24.3	26.1
Contract Revenue | Apex
Revenues:
Revenues	569.1	613.0	1,185.0	1,205.2
Contract Revenue | Oxford
Revenues:
Revenues	107.3	131.4	235.4	260.8
Permanent placement | Apex
Revenues:
Revenues	7.8	15.5	21.0	29.4
Permanent placement | Oxford
Revenues:
Revenues	12.9	21.8	33.5	42.0
Fixed-price Contract | ECS
Revenues:
Revenues	63.5	46.1	120.5	89.4
Time and materials Contract | ECS
Revenues:
Revenues	81.4	70.8	153.8	132.2
Cost-plus-fixed-fee Contract | ECS
Revenues:
Revenues	$ 94.8	$ 73.7	$ 178.1	$ 137.0