Segment Information (Detail) - Information About Reportable Segments - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating income	$ 287,997	$ 286,162	$ 275,492
Operating Segments
Segment Reporting Information [Line Items]
Revenues	2,163,056	2,021,441	1,784,213
Total segment gross contribution	1,334,547	1,234,229	1,078,165
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	1,583,486	1,445,338	1,271,011
Total segment gross contribution	1,096,827	1,001,914	879,384
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	327,735	348,396	314,257
Total segment gross contribution	107,193	119,931	107,565
Operating Segments | Events
Segment Reporting Information [Line Items]
Revenues	251,835	227,707	198,945
Total segment gross contribution	130,527	112,384	91,216
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Corporate and other expenses	$ (1,046,550)	$ (948,067)	$ (802,673)