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Goodwill and Intangible Assets - Changes to the Carrying Amount of Goodwill by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Beginning Balance	$ 2,937,726	$ 2,923,136
Additions due to acquisition		19,293
Foreign currency translation impact and other	7,821	(4,703)
Ending Balance	2,945,547	2,937,726
Research
Goodwill [Roll Forward]
Beginning Balance	2,651,060	2,638,418
Additions due to acquisition		17,557
Foreign currency translation impact and other	13,672	(4,915)
Ending Balance	2,664,732	2,651,060
Conferences
Goodwill [Roll Forward]
Beginning Balance	189,641	187,654
Additions due to acquisition		1,736
Foreign currency translation impact and other	(5,550)	251
Ending Balance	184,091	189,641
Consulting
Goodwill [Roll Forward]
Beginning Balance	97,025	97,064
Additions due to acquisition		0
Foreign currency translation impact and other	(301)	(39)
Ending Balance	$ 96,724	$ 97,025

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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