Segment Information (Detail) - Information About Reportable Segments - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating income	$ 305,141	$ 287,997	$ 286,162
Operating Segments
Segment Reporting Information [Line Items]
Revenues	2,444,540	2,163,056	2,021,441
Total segment gross contribution	1,512,000	1,334,547	1,234,229
Operating Segments \| Research
Segment Reporting Information [Line Items]
Revenues	1,829,721	1,583,486	1,445,338
Total segment gross contribution	1,267,760	1,096,827	1,001,914
Operating Segments \| Consulting
Segment Reporting Information [Line Items]
Revenues	346,214	327,735	348,396
Total segment gross contribution	107,585	107,193	119,931
Operating Segments \| Events
Segment Reporting Information [Line Items]
Revenues	268,605	251,835	227,707
Total segment gross contribution	136,655	130,527	112,384
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Corporate and other expenses	$ (1,206,859)	$ (1,046,550)	$ (948,067)

Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1,2)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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