Acquisitions Acquisition Costs, by Acquisition (Details) - USD ($) $ in Thousands				12 Months Ended
	Aug. 08, 2017	Jun. 05, 2015	Apr. 14, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Stock Issued During Period, Value, Acquisitions						$ 30,183
Stock Issued During Period, Shares, Acquisitions						794,700
Payments of Merger Related Costs, Financing Activities				$ 0	$ 16,814	$ 0
Other Payments to Acquire Businesses				0	4,780	0
Stratacuity [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Aug. 08, 2017
Business Acquisition, Name of Acquired Entity	StratAcuity Staffing Partners, Inc.
Business Acquisition, Description of Acquired Entity	headquartered in New Hampshire
Business Combination, Reason for Business Combination	was purchased to expand the Company's specialized clinical/scientific staffing solutions for biotechnology and pharmaceutical organizations
Payments to Acquire Businesses, Gross	$ 25,900
Business Combination, Acquisition Related Costs				500
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual				7,500
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual				$ 600
Business Acquisition, Goodwill, Expected Tax Deductible Amount	17,500
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 7,600
LabResource [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition			Apr. 14, 2015
Business Acquisition, Name of Acquired Entity			LabResource B.V.
Business Acquisition, Description of Acquired Entity			headquartered in Amsterdam, the Netherlands
Business Combination, Reason for Business Combination			was purchased to expand the Company's life sciences staffing business in Europe
Payments to Acquire Businesses, Gross			$ 12,700
Business Combination, Acquisition Related Costs						400
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual						7,700
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual						1,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount			6,400
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			$ 7,500
Creative Circle [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Jun. 05, 2015
Business Acquisition, Name of Acquired Entity		Creative Circle, LLC
Business Acquisition, Description of Acquired Entity		is headquartered in Los Angeles, California
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
Stock Issued During Period, Value, Acquisitions		$ 30,200
Stock Issued During Period, Shares, Acquisitions		794,700
Business Acquisition Contingent Consideration At Fair Value Accrued Earn-Out		$ 13,800
Business Combination, Acquisition Related Costs						5,700
BusinessCombinationRecognizedGoodwill		358,029
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual						167,200
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual						$ 22,900
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Payments					15,800
Payments of Merger Related Costs, Financing Activities					13,800
Other Payments to Acquire Businesses					$ 2,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount		358,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		$ 194,500