Segment Reporting (Segment Reporting Information) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual		$ 4,310		$ 8,010
Revenues:
Revenues		906,449	$ 667,048	2,470,131	$ 1,946,889
Gross Profit:
Total Gross Profit		270,172	218,315	751,755	629,396
Operating Income (Loss):
Total Operating Income		74,297	60,870	185,671	163,939
Amortization of Intangible Assets		18,540	8,248	44,689	25,011
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual		164,000		319,100
Apex
Revenues:
Revenues		589,717	517,492	1,695,728	1,502,462
Gross Profit:
Total Gross Profit		177,831	155,783	506,146	445,925
Operating Income (Loss):
Total Operating Income		71,443	59,016	192,610	162,679
Amortization of Intangible Assets		6,546	7,194	19,638	21,983
Oxford
Revenues:
Revenues		152,701	149,556	455,222	444,427
Gross Profit:
Total Gross Profit		62,712	62,532	187,321	183,471
Operating Income (Loss):
Total Operating Income		15,156	16,088	39,794	39,525
Amortization of Intangible Assets		1,040	1,054	3,143	3,028
ECS
Revenues:
Revenues		164,031	0	319,181	0
Gross Profit:
Total Gross Profit		29,629	0	58,288	0
Operating Income (Loss):
Total Operating Income			0	8,012	0
Amortization of Intangible Assets		10,954	0	21,908	0
Corporate
Operating Income (Loss):
Total Operating Income	[1]	(16,616)	(14,234)	(54,745)	(38,265)
Assignment | Apex
Revenues:
Revenues		575,309	506,376	1,653,835	1,468,976
Assignment | Oxford
Revenues:
Revenues		130,365	127,963	386,068	379,892
Permanent placement | Apex
Revenues:
Revenues		14,408	11,116	41,893	33,486
Permanent placement | Oxford
Revenues:
Revenues		22,336	21,593	69,154	64,535
Fixed-price Contract | ECS
Revenues:
Revenues		48,272	0	93,034	0
Time-and-materials Contract | ECS
Revenues:
Revenues		42,102	0	91,097	0
Cost-plus-fixed-fee Contract | ECS
Revenues:
Revenues		$ 73,657	$ 0	$ 135,050	$ 0

[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.