Acquisitions and Divestiture (Details) $ in Thousands					3 Months Ended	12 Months Ended
	Apr. 05, 2017 USD ($) employee country	Mar. 09, 2017 USD ($) employee	Nov. 09, 2016 USD ($) employee	Jun. 28, 2016 USD ($) employee	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) country	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Percentage of voting interests acquired						100.00%
Consideration transferred, aggregate purchase price						$ 3,653,028
Consideration transferred, cash paid						2,821,903
Fair value of Gartner equity						$ 818,660
Number of countries in which entity operates | country						100
Net cash paid for acquisition						$ 2,634,809	$ 34,186	$ 196,229
Acquisition and integration charges						158,450	42,598	26,175
Goodwill						2,987,294	738,453	$ 715,359
CEB
Business Acquisition [Line Items]
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, purchase accounting adjustments						$ 32,000
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
Contingent consideration arrangements, maximum value		$ 20,800
Contingent consideration arrangements, maximum value, period of expense recognition		3 years
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
Nubera eBusiness S.L. and Capterra, Inc.
Business Acquisition [Line Items]
Percentage of voting interests acquired								100.00%
Consideration transferred, cash paid								$ 206,900
Net cash paid for acquisition								196,200
Goodwill								138,100
Finite-lived intangible assets								79,600
Liabilities assumed								$ 10,800