Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended	6 Months Ended
	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill impairment	$ 0
Goodwill [Roll Forward]
Beginning balance		$ 2,923,136,000
Foreign currency translation impact		7,161,000
Ending balance		2,930,297,000
Goodwill, accumulated impairment losses			$ 0
Research
Goodwill [Roll Forward]
Beginning balance		2,638,418,000
Foreign currency translation impact		7,194,000
Ending balance		2,645,612,000
Conferences
Goodwill [Roll Forward]
Beginning balance		187,654,000
Foreign currency translation impact		16,000
Ending balance		187,670,000
Consulting
Goodwill [Roll Forward]
Beginning balance		97,064,000
Foreign currency translation impact		(49,000)
Ending balance		$ 97,015,000