Acquisitions and Divestiture - Summary of Consideration Transferred (Details) - USD ($) $ in Thousands			3 Months Ended	12 Months Ended
	Apr. 05, 2017	Mar. 09, 2017	Sep. 30, 2017	Dec. 31, 2017
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
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