Acquisitions - Summary of Consideration Transferred (Details) - USD ($) $ in Thousands			1 Months Ended
	Apr. 05, 2017	Mar. 09, 2017	Apr. 05, 2017
Business Acquisition [Line Items]
Aggregate consideration			$ 2,821,903
Total aggregate consideration			$ 3,653,028
CEB
Business Acquisition [Line Items]
Aggregate consideration	$ 2,687,704
Additional cash to be paid	12,465
Fair value of Gartner equity awards	818,660
Total aggregate consideration	$ 3,518,829
L2, Inc.
Business Acquisition [Line Items]
Aggregate consideration		$ 134,199
Additional cash to be paid		20,800
Total aggregate consideration		$ 134,199