Segment Information (Detail) - Information About Reportable Segments (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$ 2,021,441us-gaap_Revenues	$ 1,784,213us-gaap_Revenues	$ 1,615,808us-gaap_Revenues
Total segment gross contribution	1,234,229us-gaap_GrossProfit	1,078,165us-gaap_GrossProfit	963,714us-gaap_GrossProfit
Corporate and other expenses	(948,067)us-gaap_OperatingExpenses	(802,673)us-gaap_OperatingExpenses	(718,007)us-gaap_OperatingExpenses
Operating income	286,162us-gaap_OperatingIncomeLoss	275,492us-gaap_OperatingIncomeLoss	245,707us-gaap_OperatingIncomeLoss
Research
Segment Reporting Information [Line Items]
Revenues	1,445,338	1,271,011	1,137,147
Total segment gross contribution	1,001,914	879,384	774,342
Consulting
Segment Reporting Information [Line Items]
Revenues	348,396	314,257	304,893
Total segment gross contribution	119,931	107,565	109,253
Events
Segment Reporting Information [Line Items]
Revenues	227,707	198,945	173,768
Total segment gross contribution	$ 112,384	$ 91,216	$ 80,119

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

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- Definition

Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

+ References

No definition available.

+ Details

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

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- Definition

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=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

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- Details

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- Details

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