Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill impairment	$ 0
Goodwill [Roll Forward]
Beginning balance		$ 2,923,136,000
Foreign currency translation impact		16,571,000
Ending balance	2,939,707,000	2,939,707,000
Goodwill, accumulated impairment losses			$ 0
Research
Goodwill [Roll Forward]
Beginning balance		2,638,418,000
Foreign currency translation impact		16,745,000
Ending balance	2,655,163,000	2,655,163,000
Conferences
Goodwill [Roll Forward]
Beginning balance		187,654,000
Foreign currency translation impact		24,000
Ending balance	187,678,000	187,678,000
Consulting
Goodwill [Roll Forward]
Beginning balance		97,064,000
Foreign currency translation impact		(198,000)
Ending balance	$ 96,866,000	$ 96,866,000