Segment Information (Details) - Information About Reportable Segments	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) segment	Jun. 30, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3
Revenues	$ 609,998,000	$ 547,936,000	$ 1,167,264,000	$ 1,019,122,000
Operating income	83,299,000	85,220,000	147,728,000	133,902,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	609,998,000	547,936,000	1,167,264,000	1,019,122,000
Gross contribution	383,024,000	342,543,000	733,570,000	638,377,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(299,725,000)	(257,323,000)	(585,842,000)	(504,475,000)
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	449,170,000	385,718,000	889,441,000	766,808,000
Gross contribution	313,688,000	269,846,000	621,874,000	536,911,000
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	94,068,000	88,336,000	179,008,000	165,128,000
Gross contribution	33,274,000	33,255,000	62,651,000	59,582,000
Events | Reportable segments
Segment Reporting Information [Line Items]
Revenues	66,760,000	73,882,000	98,815,000	87,186,000
Gross contribution	$ 36,062,000	$ 39,442,000	$ 49,045,000	$ 41,884,000