Business Segments Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Revenues	$ 929,650	$ 906,449	$ 878,509	$ 685,173	$ 679,035	$ 667,048	$ 653,313	$ 626,528	$ 3,399,781	$ 2,625,924	$ 2,440,413
Apex
Revenue from External Customer [Line Items]
Revenues									2,300,242	2,037,154	1,836,488
Apex | Assignment [Member]
Revenue from External Customer [Line Items]
Revenues									2,244,470	1,993,256	1,791,566
Apex | PermanentPlacement [Member]
Revenue from External Customer [Line Items]
Revenues									55,772	43,898	44,922
Oxford
Revenue from External Customer [Line Items]
Revenues									606,510	588,770	603,925
Oxford | Assignment [Member]
Revenue from External Customer [Line Items]
Revenues									516,045	503,058	520,705
Oxford | PermanentPlacement [Member]
Revenue from External Customer [Line Items]
Revenues									90,465	85,712	83,220
ECS
Revenue from External Customer [Line Items]
Revenues									493,029	0	0
ECS | Fixed-price Contract [Member]
Revenue from External Customer [Line Items]
Revenues									133,146	0	0
ECS | Time-and-materials Contract [Member]
Revenue from External Customer [Line Items]
Revenues									143,434	0	0
ECS | Cost-plus-fixed-fee Contract [Member]
Revenue from External Customer [Line Items]
Revenues									$ 216,449	$ 0	$ 0