Acquisitions - (Details)			12 Months Ended
	Nov. 09, 2016 USD ($) employee	Jun. 28, 2016 USD ($) employee	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) company
Business Acquisition [Line Items]
Number of Businesses Acquired | company						3
Goodwill				$ 738,453,000	$ 715,359,000	$ 586,665,000
Net cash paid for acquisition				34,186,000	$ 196,229,000	$ 124,291,000
Market-Visio Oy
Business Acquisition [Line Items]
Percentage of voting interests acquired					100.00%
SircleIT Inc.
Business Acquisition [Line Items]
Percentage of voting interests acquired					100.00%
Machina Research
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%
Entity number of employees | employee	16
Aggregate purchase price	$ 4,500,000
SCM World
Business Acquisition [Line Items]
Percentage of voting interests acquired		100.00%
Entity number of employees | employee		60
Aggregate purchase price		$ 34,200,000
Goodwill		32,400,000
Net cash paid for acquisition		27,900,000
Business combination, amortizable intangible assets recognized		5,900,000
Nubera and Capterra
Business Acquisition [Line Items]
Percentage of voting interests acquired					100.00%
Aggregate purchase price					$ 206,900,000
Business combinations, payables and accrueds recognized					25,600,000
Goodwill					138,100,000
Business combination, liabilities assumed					10,800,000
Net cash paid for acquisition					196,200,000
Contingent consideration arrangements, maximum value (up to)					$ 32,000,000
Contingent consideration arrangements, maximum value, period of expense recognition					3 years
Business combination, amortizable intangible assets recognized					$ 79,600,000
Capterra
Business Acquisition [Line Items]
Goodwill, purchase accounting adjustments				1,900,000
Software Advice
Business Acquisition [Line Items]
Percentage of voting interests acquired						100.00%
Software Advice, Inc., Market-Visio Oy, and SircleIT Inc.
Business Acquisition [Line Items]
Aggregate purchase price						$ 115,400,000
Net cash paid for acquisition						109,900,000
Acquisition cash place in escrow						$ 14,400,000
Contingent consideration arrangements, maximum value, period of expense recognition						2 years
Change in contingent consideration				(22,700,000)	(9,200,000)
Other payments to acquire businesses, restricted cash				13,600,000	$ 800,000
Business combination, goodwill and amortizable intangible assets recognized						$ 110,300,000
Business combinations, other assets recognized						5,100,000
Contingent consideration, earn-out | SCM World
Business Acquisition [Line Items]
Change in contingent consideration				$ 6,500,000
Contingent consideration, employee conditions | SCM World
Business Acquisition [Line Items]
Contingent consideration arrangements, maximum value (up to)		$ 5,400,000
Contingent consideration arrangements, maximum value, period of expense recognition		2 years
Acquisition and Integration Charges | Software Advice, Inc., Market-Visio Oy, and SircleIT Inc.
Business Acquisition [Line Items]
Contingent consideration arrangements, maximum value (up to)						$ 31,900,000
Scenario, Forecast | Capterra
Business Acquisition [Line Items]
Other payments to acquire businesses, restricted cash			$ 25,600,000