Acquisitions and Divestiture - Additional Information (Details) $ in Thousands					3 Months Ended		12 Months Ended			21 Months Ended
	Apr. 05, 2017 USD ($) employee country	Mar. 09, 2017 USD ($) employee	Nov. 09, 2016 USD ($) employee	Jun. 28, 2016 USD ($) employee	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) country	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) country
Business Acquisition [Line Items]
Acquisition and integration charges							$ 107,197	$ 158,450	$ 42,598
Percentage of voting interests acquired					100.00%			100.00%
Number of countries in which entity operates | country							100			100
Net cash paid for acquisition					$ 2,640,000		$ 15,855	$ 2,641,780	48,196
Goodwill					$ 2,987,294		2,923,136	2,987,294	738,453	$ 2,923,136
CEB
Business Acquisition [Line Items]
Charges and adjustments							69,790	13,087		82,900
Payments							26,087	126		26,200
Exit costs							90,600			$ 90,600
Percentage of voting interests acquired	100.00%
Consideration transferred, aggregate purchase price	$ 3,518,829
Consideration transferred, cash paid	2,687,704					$ 12,500
Fair value of Gartner equity	$ 818,660
Entity number of employees | employee	4,900
Number of countries in which entity operates | country	70
Proceeds from issuance of debt	$ 2,800,000
Goodwill	2,349,589
Finite-lived intangible assets	1,584,300
Accounts payable and accrued liabilities	142,134
Liabilities assumed	$ 957,033
L2, Inc.
Business Acquisition [Line Items]
Percentage of voting interests acquired		100.00%
Consideration transferred, aggregate purchase price		$ 134,199
Consideration transferred, cash paid		134,199
Fair value of Gartner equity		$ 0
Entity number of employees | employee		150
Goodwill		$ 108,202
Finite-lived intangible assets		15,890
Accounts payable and accrued liabilities		3,050
Liabilities assumed		$ 17,919
Machina Research Limited And SCM World
Business Acquisition [Line Items]
Goodwill, purchase accounting adjustments									1,900
Goodwill									32,400
Finite-lived intangible assets									$ 5,900
Machina Research
Business Acquisition [Line Items]
Percentage of voting interests acquired			100.00%
Consideration transferred, cash paid			$ 4,500
Entity number of employees | employee			16
SCM World
Business Acquisition [Line Items]
Percentage of voting interests acquired				100.00%
Consideration transferred, cash paid				$ 34,200
Entity number of employees | employee				60
Net cash paid for acquisition				$ 27,900
Disposed of by Sale | CEB
Business Acquisition [Line Items]
Charges and adjustments							$ 7,500	$ (10,100)