Business Segments Disaggregated Revenue (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
Revenue from External Customer [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
Apex
Revenue from External Customer [Line Items]
Revenues										2,520.0	2,300.3	2,037.2
Apex | Assignment [Member]
Revenue from External Customer [Line Items]
Revenues										2,463.6	2,244.5	1,993.3
Apex | PermanentPlacement [Member]
Revenue from External Customer [Line Items]
Revenues										56.4	55.8	43.9
Oxford
Revenue from External Customer [Line Items]
Revenues										605.7	606.5	588.8
Oxford | Assignment [Member]
Revenue from External Customer [Line Items]
Revenues										522.7	516.0	503.1
Oxford | PermanentPlacement [Member]
Revenue from External Customer [Line Items]
Revenues										83.0	90.5	85.7
ECS
Revenue from External Customer [Line Items]
Revenues										798.2	493.0	0.0
ECS | Fixed-price Contract [Member]
Revenue from External Customer [Line Items]
Revenues										214.0	133.1	0.0
ECS | Time-and-materials Contract [Member]
Revenue from External Customer [Line Items]
Revenues										267.8	143.4	0.0
ECS | Cost-plus-fixed-fee Contract [Member]
Revenue from External Customer [Line Items]
Revenues										$ 316.4	$ 216.5	$ 0.0

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