Business Segments (Segment Reporting Information) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) property	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment Reporting Information
Revenues	$ 577,517	$ 572,123	$ 485,323	$ 430,045	$ 441,023	$ 442,443	$ 434,424	$ 406,851	$ 2,065,008	$ 1,724,741	$ 1,523,101
Gross profit	$ 192,932	$ 191,404	$ 158,534	$ 135,875	$ 142,537	$ 144,838	$ 141,905	$ 128,155	678,745	557,435	454,875
Operating Income									152,108	137,782	116,587
Amortization of intangible assets									$ 34,467	22,130	20,943
Reportable segments (in number of segments) | property									2
Apex
Segment Reporting Information
Revenues									$ 1,487,042	1,190,052	1,059,993
Gross profit									437,507	335,322	294,611
Operating Income									151,559	121,334	98,588
Amortization of intangible assets									28,371	16,356	19,524
Oxford
Segment Reporting Information
Revenues									577,966	534,689	463,108
Gross profit									241,238	222,113	160,264
Operating Income									70,385	70,193	66,118
Amortization of intangible assets									6,096	5,765	1,329
Corporate
Segment Reporting Information
Operating Income									(69,836)	(53,745)	(48,119)
Amortization of intangible assets									0	9	90
Assignment [Member]
Segment Reporting Information
Revenues									1,947,001	1,641,706	1,500,157
Direct hire and conversion fees [Member]
Segment Reporting Information
Revenues									$ 118,007	$ 83,035	$ 22,944