Business Segments (Segment Reporting Information) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information
Revenues	$ 620,884	$ 629,401	$ 608,088	$ 582,040	$ 577,517	$ 572,123	$ 485,323	$ 430,045	$ 2,440,413	$ 2,065,008	$ 1,724,741
Gross profit	$ 198,195	$ 207,120	$ 202,086	$ 187,782	$ 192,932	$ 191,404	$ 158,534	$ 135,875	795,183	678,745	557,435
Operating Income									189,726	152,108	137,782
Amortization of intangible assets									$ 39,628	34,467	22,130
Reportable segments (in number of segments) | property									2
Apex
Segment Reporting Information
Revenues									$ 1,836,488	1,493,608	1,198,140
Gross profit									548,421	439,586	338,023
Operating Income									195,133	151,450	121,838
Amortization of intangible assets									34,359	28,371	16,356
Oxford
Segment Reporting Information
Revenues									603,925	571,400	526,601
Gross profit									246,762	239,159	219,412
Operating Income									51,294	59,059	58,741
Amortization of intangible assets									5,269	6,096	5,765
Corporate
Segment Reporting Information
Operating Income									(56,701)	(58,401)	(42,797)
Amortization of intangible assets									0	0	9
Assignment [Member]
Segment Reporting Information
Revenues									2,312,271	1,947,001	1,641,706
PermanentPlacement [Member]
Segment Reporting Information
Revenues									$ 128,142	$ 118,007	$ 83,035