Acquisitions Acquisitions (Schedule of Intangible Assets Acquired as Part of Business Combination) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Jun. 05, 2015	Apr. 14, 2015
Contract-Based Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	5 years
Computer Software, Intangible Asset [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	6 years
Trademarks [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	2 years
Creative Circle [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		$ 194,500,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount		358,029,000
Creative Circle [Member] | Trademarks [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		66,100,000
Creative Circle [Member] | Contractor Relations [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		29,500,000
Creative Circle [Member] | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		90,700,000
Creative Circle [Member] | Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		7,300,000
Creative Circle [Member] | Contract-Based Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		$ 900,000
LabResource [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			$ 7,528,000
LabResource [Member] | Trademarks [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			1,140,000
LabResource [Member] | Contractor Relations [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			947,000
LabResource [Member] | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			5,421,000
LabResource [Member] | Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			20,000
LabResource [Member] | Contract-Based Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			$ 0
Minimum [Member] | Contractor Relations [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	2 years
Minimum [Member] | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	2 years
Minimum [Member] | Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	3 years
Maximum [Member] | Contractor Relations [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	4 years
Maximum [Member] | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	10 years
Maximum [Member] | Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Assets Average Useful Life	6 years