Business Segments Segment Reporting Information (Details) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)		Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)
Segment Reporting Information
Revenues	$ 679,035	$ 667,048	$ 653,313	$ 626,528	$ 620,884		$ 629,401	$ 608,088	$ 582,040	$ 2,625,924	$ 2,440,413		$ 2,065,008
Gross profit	$ 220,677	$ 218,315	$ 212,937	$ 198,144	$ 198,195	[1]	$ 207,120	$ 202,086	$ 187,782	850,073	795,183		678,745
Operating Income										224,736	189,726		152,108
Amortization of intangible assets										$ 33,444	39,628		34,467
Reportable segments (in number of segments) | property										2
Apex
Segment Reporting Information
Revenues										$ 2,037,154	1,836,488		1,493,608
Gross profit										606,294	548,421		439,586
Operating Income										222,010	195,133		151,450
Amortization of intangible assets										29,361	34,359		28,371
Oxford
Segment Reporting Information
Revenues										588,770	603,925		571,400
Gross profit										243,779	246,762		239,159
Operating Income										53,787	51,294		59,059
Amortization of intangible assets										4,083	5,269		6,096
Corporate
Segment Reporting Information
Operating Income										(51,061)	(56,701)	[2]	(58,401)	[2]
Amortization of intangible assets										0	0		0
Assignment [Member]
Segment Reporting Information
Revenues										2,496,314	2,312,271		1,947,001
PermanentPlacement [Member]
Segment Reporting Information
Revenues										$ 129,610	$ 128,142		$ 118,007

[1] The fourth quarter of 2016 included out-of-period adjustments of $5.6 million for costs of services that were understated in prior quarters. As a result, costs of services for the fourth quarter of 2016 were overstated by $5.6 million and gross profit, net income and diluted earnings per common share were understated by $5.6 million, $3.4 million and $0.06, respectively.

[2] 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.