Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 973,135	$ 1,070,882	$ 1,992,026	$ 2,041,326
Research
Disaggregation of Revenue [Line Items]
Total revenues	875,329	826,055	1,784,620	1,651,429
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	317	141,174	14,187	193,106
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	97,489	103,653	193,219	196,791
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	631,579	690,556	1,281,878	1,301,889
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	573,245	528,461	1,163,400	1,055,694
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	317	100,596	6,297	129,603
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	58,017	61,499	112,181	116,592
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	226,968	253,441	465,137	494,527
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	196,854	193,666	402,794	387,621
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	0	25,827	2,147	43,024
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	30,114	33,948	60,196	63,882
Other International
Disaggregation of Revenue [Line Items]
Total revenues	114,588	126,885	245,011	244,910
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	105,230	103,928	218,426	208,114
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	0	14,751	5,743	20,479
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	$ 9,358	$ 8,206	$ 20,842	$ 16,317