Acquisitions - Summary of Consideration Transferred (Details) - USD ($) $ in Thousands			1 Months Ended	3 Months Ended
	Apr. 05, 2017	Mar. 09, 2017	Apr. 05, 2017	Sep. 30, 2017
Business Acquisition [Line Items]
Cash paid at close			$ 2,821,903
Additional cash paid			12,465
Fair value of Gartner equity awards		$ 0	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 awards	818,660
Total aggregate consideration	$ 3,518,829
L2, Inc.
Business Acquisition [Line Items]
Cash paid at close		134,199
Additional cash paid		0
Total aggregate consideration		$ 134,199