Acquisitions Acquisitions Costs, by Acquisition (Details) $ in Thousands, € in Millions				3 Months Ended		6 Months Ended
	Jun. 05, 2015 USD ($)	Apr. 14, 2015 USD ($)	Apr. 14, 2015 EUR (€)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) shares	Jun. 30, 2014 USD ($)
Business Acquisition [Line Items]
Write off of Deferred Debt Issuance Cost					$ 0	$ 3,751	$ 0
Business Combination, Acquisition Related Costs						6,100
Potential future earn-out obligations				$ 41,000		41,000
Stock Issued During Period, Value, Acquisitions						$ 30,200
Stock Issued During Period, Shares, Acquisitions | shares						794,700
Sales Revenue, Services, Net				485,323	434,424	$ 915,368	841,275
Operating Income (Loss)				$ 32,710	$ 36,769	57,781	$ 63,277
Creative Circle [Member]
Business Acquisition [Line Items]
BusinessCombinationRecognizedGoodwill	$ 361,746
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	194,500
Business Acquisition, Goodwill, Expected Tax Deductible Amount	$ 348,900
Business Combination, Acquisition Related Costs						5,700
Business Acquisition, Effective Date of Acquisition	Jun. 05, 2015
Potential future earn-out obligations	$ 30,000
Business Acquisition, Name of Acquired Entity	Creative Circle, LLC
Business Acquisition, Description of Acquired Entity	is headquartered in Los Angeles, California
Business Combination, Consideration Transferred	$ 540,900
Business Combination, Consideration Transferred, Other	$ 900
Business Combination, Reason for Business Combination	was purchased to expand the Company’s technical and creative staffing services
Payments to Acquire Businesses, Gross	$ 540,000
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned	30,000
Sales Revenue, Services, Net						19,600
Operating Income (Loss)						2,900
LabResource [Member]
Business Acquisition [Line Items]
BusinessCombinationRecognizedGoodwill		$ 6,104
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		$ 7,528
Business Combination, Acquisition Related Costs						400
Business Acquisition, Effective Date of Acquisition		Apr. 14, 2015	Apr. 14, 2015
Business Acquisition, Name of Acquired Entity		LabResource B.V.	LabResource B.V.
Business Acquisition, Description of Acquired Entity		headquartered in Amsterdam, Netherlands	headquartered in Amsterdam, Netherlands
Business Combination, Consideration Transferred		$ 11,900
Business Combination, Consideration Transferred, Other | €			€ 0.8
Business Combination, Reason for Business Combination		expand the Company's life sciences staffing business in Europe	expand the Company's life sciences staffing business in Europe
Payments to Acquire Businesses, Gross		$ 12,700
Sales Revenue, Services, Net						2,200
Operating Income (Loss)						$ 300
Euro Member Countries, Euro | LabResource [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred | €			€ 11.3
Business Combination, Consideration Transferred, Other | €			€ 0.7
Payments to Acquire Businesses, Gross		12,000
Contractor Relations [Member]
Business Acquisition [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life						4 years
Contractor Relations [Member] | Creative Circle [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	29,500
Contractor Relations [Member] | LabResource [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		947
Non-compete agreements | Creative Circle [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	7,300
Non-compete agreements | LabResource [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		20
Contract-Based Intangible Assets [Member]
Business Acquisition [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life						5 years
Contract-Based Intangible Assets [Member] | Creative Circle [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	900
Contract-Based Intangible Assets [Member] | LabResource [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		0
Customer relations
Business Acquisition [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life						10 years
Customer relations | Creative Circle [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 90,700
Customer relations | LabResource [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		$ 5,421
Maximum [Member] | Non-compete agreements
Business Acquisition [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life						6 years
Minimum [Member] | Non-compete agreements
Business Acquisition [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life						2 years