Acquisitions - Proforma (Details) - CEB - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Business Acquisition, Pro Forma Information [Abstract]
Pro forma total revenue	$ 891,003	$ 756,083	$ 2,664,450	$ 2,233,942
Pro forma net (loss)	$ (7,283)	$ (40,825)	$ (71,122)	$ (133,379)
Pro forma basic earnings per share (in dollars per share)	$ (0.08)	$ (0.45)	$ (0.79)	$ (1.48)
Pro forma diluted earnings per share (in dollars per share)	$ (0.08)	$ (0.45)	$ (0.79)	$ (1.48)

The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)-(3)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

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