Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Sep. 30, 2018	Dec. 31, 2018
Goodwill [Line Items]
Goodwill impairment		$ 0
Goodwill [Roll Forward]
Beginning balance	$ 2,923,136,000
Foreign currency translation impact	(16,400,000)
Ending balance	2,906,736,000
Goodwill, accumulated impairment losses			$ 0
Research
Goodwill [Roll Forward]
Beginning balance	2,638,418,000
Foreign currency translation impact	(16,386,000)
Ending balance	2,622,032,000
Conferences
Goodwill [Roll Forward]
Beginning balance	187,654,000
Foreign currency translation impact	(61,000)
Ending balance	187,593,000
Consulting
Goodwill [Roll Forward]
Beginning balance	97,064,000
Foreign currency translation impact	47,000
Ending balance	$ 97,111,000