Acquisitions and Divestiture - Proforma (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Business Acquisition, Pro Forma Information [Abstract]
Pro forma diluted income (loss) per share (in dollars per share)	$ 0.76	$ (1.84)
CEB
Business Acquisition, Pro Forma Information [Abstract]
Pro forma total revenue	$ 3,726,470	$ 3,183,070
Pro forma net income (loss)	$ 150,167	$ (241,423)
Pro forma basic income (loss) per share (in dollars per share)	$ 1.66	$ (2.68)

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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