Goodwill and Intangible Assets - Changes to the Carrying Amount of Goodwill by Segment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Beginning Balance	$ 2,923,136,000	$ 2,987,294,000
Divestitures		(92,578,000)
Additions due to acquisition	19,293,000
Foreign currency translation impact and other	(4,703,000)	28,420,000
Ending Balance	2,937,726,000	2,923,136,000
Accumulated goodwill impairment losses	0
Research
Goodwill [Roll Forward]
Beginning Balance	2,638,418,000	2,619,677,000
Divestitures		(2,500,000)
Additions due to acquisition	17,557,000
Foreign currency translation impact and other	(4,915,000)	21,241,000
Ending Balance	2,651,060,000	2,638,418,000
Conferences
Goodwill [Roll Forward]
Beginning Balance	187,654,000	187,920,000
Divestitures		0
Additions due to acquisition	1,736,000
Foreign currency translation impact and other	251,000	(266,000)
Ending Balance	189,641,000	187,654,000
Consulting
Goodwill [Roll Forward]
Beginning Balance	97,064,000	97,798,000
Divestitures		0
Additions due to acquisition	0
Foreign currency translation impact and other	(39,000)	(734,000)
Ending Balance	97,025,000	97,064,000
Other
Goodwill [Roll Forward]
Beginning Balance	0	81,899,000
Divestitures		(90,078,000)
Additions due to acquisition	0
Foreign currency translation impact and other	0	8,179,000
Ending Balance	$ 0	$ 0