Business and Significant Accounting Policies - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenues	$ 3,975,454	$ 3,311,494	$ 2,444,540
Research
Disaggregation of Revenue [Line Items]
Total revenues	3,105,764	2,471,280	1,857,001
Conferences
Disaggregation of Revenue [Line Items]
Total revenues	410,461	337,903	268,605
Consulting
Disaggregation of Revenue [Line Items]
Total revenues	353,667	327,661	318,934
Other
Disaggregation of Revenue [Line Items]
Total revenues	105,562	174,650	0
United States and Canada
Disaggregation of Revenue [Line Items]
Total revenues	2,514,952	2,092,366	1,519,748
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Total revenues	1,994,016	1,600,847	1,178,575
United States and Canada | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	256,219	210,698	162,162
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	205,874	188,022	179,011
United States and Canada | Other
Disaggregation of Revenue [Line Items]
Total revenues	58,843	92,799	0
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenues	1,000,490	855,421	616,721
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Total revenues	737,129	597,943	434,753
Europe, Middle East and Africa | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	105,909	86,567	72,926
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	119,258	111,792	109,042
Europe, Middle East and Africa | Other
Disaggregation of Revenue [Line Items]
Total revenues	38,194	59,119	0
Other International
Disaggregation of Revenue [Line Items]
Total revenues	460,012	363,707	308,071
Other International | Research
Disaggregation of Revenue [Line Items]
Total revenues	374,619	272,490	243,673
Other International | Conferences
Disaggregation of Revenue [Line Items]
Total revenues	48,333	40,638	33,517
Other International | Consulting
Disaggregation of Revenue [Line Items]
Total revenues	28,535	27,847	30,881
Other International | Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 8,525	$ 22,732	$ 0