Acquisitions and Divestiture - Additional Information (Details)				3 Months Ended	12 Months Ended
	Oct. 01, 2019 USD ($)	Apr. 05, 2017 USD ($) employee country	Mar. 09, 2017 USD ($) employee	Sep. 30, 2017 USD ($)	Dec. 31, 2019 USD ($) country	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Percentage of voting interests acquired							100.00%
Net cash paid for acquisition					$ 25,989,000	$ 15,855,000	$ 2,641,780,000
Deferred consideration					$ 2,300,000	15,900,000
Number of countries in which entity operates | country					100
Acquisition and integration charges					$ 9,463,000	107,197,000	158,450,000
Exit costs					$ 0	$ 58,300,000	$ 13,100,000
TOPO
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%
Aggregate purchase price	$ 25,000,000.0
Net cash paid for acquisition	23,700,000
Future compensation expense (up to)	$ 6,500,000
CEB
Business Acquisition [Line Items]
Percentage of voting interests acquired		100.00%
Aggregate purchase price		$ 3,518,829,000
Consideration transferred, cash paid		2,687,704,000		$ 12,500,000
Fair value of Gartner equity		$ 818,660,000
Entity number of employees | employee		4,900
Number of countries in which entity operates | country		70
L2, Inc.
Business Acquisition [Line Items]
Percentage of voting interests acquired			100.00%
Aggregate purchase price			$ 134,199,000
Consideration transferred, cash paid			134,199,000
Fair value of Gartner equity			$ 0
Entity number of employees | employee			150