Business and Basis of Presentation - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	$ 963,565	$ 625,169
Research
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	763,924	511,306
Events
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	46,087	35,269
Consulting
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	82,896	78,594
Talent Assessment & Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	70,658	0
United States and Canada
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	593,505	392,987
United States and Canada | Research
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	489,713	327,492
United States and Canada | Events
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	24,069	17,663
United States and Canada | Consulting
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	45,129	47,832
United States and Canada | Talent Assessment & Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	34,594	0
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	259,666	153,720
Europe, Middle East and Africa | Research
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	184,547	115,802
Europe, Middle East and Africa | Events
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	16,891	14,095
Europe, Middle East and Africa | Consulting
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	29,938	23,823
Europe, Middle East and Africa | Talent Assessment & Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	28,290	0
Other International
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	110,394	78,462
Other International | Research
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	89,664	68,012
Other International | Events
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	5,127	3,511
Other International | Consulting
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	7,829	6,939
Other International | Talent Assessment & Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customer	$ 7,774	$ 0