Acquisitions - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Acquisitions, net of cash acquired		$ 0	$ 0	$ 74,959
Infostrada, SportsDirect, Covers, Enswers
Business Acquisition [Line Items]
Acquisitions, net of cash acquired	$ 69,849
Transaction costs incurred	$ 3,000
Definite-lived intangible assets useful life, years	12 years
Infostrada, SportsDirect, Covers, Enswers | Customer relationships
Business Acquisition [Line Items]
Definite-lived intangible assets useful life, years	11 years
Infostrada, SportsDirect, Covers, Enswers | Content database
Business Acquisition [Line Items]
Definite-lived intangible assets useful life, years	14 years
Infostrada, SportsDirect, Covers, Enswers | Technologies
Business Acquisition [Line Items]
Definite-lived intangible assets useful life, years	8 years
Infostrada, SportsDirect, Covers, Enswers | Trade names and trademarks
Business Acquisition [Line Items]
Definite-lived intangible assets useful life, years	15 years
Infostrada, SportsDirect, Covers, Enswers | Noncompete Agreements
Business Acquisition [Line Items]
Definite-lived intangible assets useful life, years	5 years