Goodwill and Intangible Assets (Details) - Changes to Carrying Amount of Goodwill by Reporting Unit - USD ($)	3 Months Ended	6 Months Ended
	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill impairment	$ 0
Goodwill, accumulated impairment losses			$ 0
Goodwill [Roll Forward]
Balance, December 31, 2016		$ 738,453,000
Additions due to acquisitions		2,427,974,000
Foreign currency translation impact		3,466,000
Balance at June 30, 2017		3,169,893,000
Research
Goodwill [Roll Forward]
Balance, December 31, 2016		595,450,000
Additions due to acquisitions		1,883,493,000
Foreign currency translation impact		4,051,000
Balance at June 30, 2017		2,482,994,000
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2016		96,480,000
Additions due to acquisitions		0
Foreign currency translation impact		469,000
Balance at June 30, 2017		96,949,000
Events
Goodwill [Roll Forward]
Balance, December 31, 2016		46,523,000
Additions due to acquisitions		148,737,000
Foreign currency translation impact		197,000
Balance at June 30, 2017		195,457,000
Talent Assessment & Other
Goodwill [Roll Forward]
Balance, December 31, 2016		0
Additions due to acquisitions		395,744,000
Foreign currency translation impact		(1,251,000)
Balance at June 30, 2017		$ 394,493,000