Business and Significant Accounting Policies - Changes to the Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Beginning Balance	$ 2,987,294	$ 738,453
Additions due to acquisition		2,457,791
Reclassified as held-for-sale		(212,994)
Foreign currency translation impact and other	28,420	4,044
Ending Balance	2,923,136	2,987,294
Not Discontinued Operations
Goodwill [Roll Forward]
Additions due to acquisitions	(92,578)
Research
Goodwill [Roll Forward]
Beginning Balance	2,619,677	595,450
Additions due to acquisition		2,042,514
Reclassified as held-for-sale		0
Foreign currency translation impact and other	21,241	(18,287)
Ending Balance	2,638,418	2,619,677
Research | Not Discontinued Operations
Goodwill [Roll Forward]
Additions due to acquisitions	(2,500)
Conferences
Goodwill [Roll Forward]
Beginning Balance	187,920	46,523
Additions due to acquisition		140,914
Reclassified as held-for-sale		0
Foreign currency translation impact and other	(266)	483
Ending Balance	187,654	187,920
Conferences | Not Discontinued Operations
Goodwill [Roll Forward]
Additions due to acquisitions	0
Consulting
Goodwill [Roll Forward]
Beginning Balance	97,798	96,480
Additions due to acquisition		0
Reclassified as held-for-sale		0
Foreign currency translation impact and other	(734)	1,318
Ending Balance	97,064	97,798
Consulting | Not Discontinued Operations
Goodwill [Roll Forward]
Additions due to acquisitions	0
Other
Goodwill [Roll Forward]
Beginning Balance	81,899	0
Additions due to acquisition		274,363
Reclassified as held-for-sale		(212,994)
Foreign currency translation impact and other	8,179	20,530
Ending Balance	0	$ 81,899
Other | Not Discontinued Operations
Goodwill [Roll Forward]
Additions due to acquisitions	$ (90,078)