Acquisitions and Divestiture - Summary of Consideration Transferred (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions			3 Months Ended	12 Months Ended
	Apr. 05, 2017	Mar. 09, 2017	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Percentage of voting interests acquired						100.00%
Net cash paid for acquisition				$ 25,989	$ 15,855	$ 2,641,780
CEB and L2 Acquisitions
Business Acquisition [Line Items]
Cash paid at close						2,821,903
Additional cash paid						12,465
Fair value of Gartner equity						818,660
Total aggregate consideration						$ 3,653,028
CEB
Business Acquisition [Line Items]
Cash paid at close	$ 2,687,704		$ 12,500
Additional cash paid	12,465
Fair value of Gartner equity	818,660
Total aggregate consideration	$ 3,518,829
Percentage of voting interests acquired	100.00%
Proceeds from issuance of debt	$ 2,800,000
Business acquisition, shares issued as consideration (in shares)	7.4
Business acquisition, consideration transferred, per share value (in dollars per share)	$ 109.65
L2, Inc.
Business Acquisition [Line Items]
Cash paid at close		$ 134,199
Additional cash paid		0
Fair value of Gartner equity		0
Total aggregate consideration		$ 134,199
Percentage of voting interests acquired		100.00%