Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2017
Goodwill [Line Items]
Goodwill impairment		$ 0
Goodwill, accumulated impairment losses			$ 0
Goodwill [Roll Forward]
Balance, December 31, 2017	$ 2,987,294,000
Reclassified as held-for-sale	(20,547,000)
Foreign currency translation impact and other	(10,105,000)
Balance at March 31, 2018	2,956,642,000
Research
Goodwill [Roll Forward]
Balance, December 31, 2017	2,619,677,000
Reclassified as held-for-sale	0
Foreign currency translation impact and other	(17,958,000)
Balance at March 31, 2018	2,601,719,000
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2017	97,798,000
Reclassified as held-for-sale	0
Foreign currency translation impact and other	528,000
Balance at March 31, 2018	98,326,000
Events
Goodwill [Roll Forward]
Balance, December 31, 2017	187,920,000
Reclassified as held-for-sale	0
Foreign currency translation impact and other	(170,000)
Balance at March 31, 2018	187,750,000
Talent Assessment & Other
Goodwill [Roll Forward]
Balance, December 31, 2017	81,899,000
Reclassified as held-for-sale	(20,547,000)
Foreign currency translation impact and other	7,495,000
Balance at March 31, 2018	$ 68,847,000