Acquisitions Acquisitions (Acquisition Costs, by Acquisition) (Details) - USD ($) $ in Thousands			12 Months Ended
	Jun. 05, 2015	Apr. 14, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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			$ 16,814	$ 0	$ 691
Other Payments to Acquire Businesses			4,780	0	$ 0
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	794,700,000
Business Acquisition Contingent Consideration At Fair Value Accrued Earn-Out	$ 13,800
Business Combination, Acquisition Related Costs				5,700
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
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, Netherlands
Business Combination, Reason for Business Combination		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