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.

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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of transfers into (out of) an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Goodwill and Other Intangible Assets - Summary of Goodwill by Segment and Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands			12 Months Ended
	May 01, 2020	Jan. 01, 2020	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill, Beginning Balance		$ 243,895	$ 243,895	$ 85,165
Acquisitions				158,569
Translation adjustments			3,316	161
Goodwill, Ending Balance			247,211	243,895
Research Segment [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance		101,311	101,311	72,647
Reassignment	$ 4,195	130,179		12,518
Acquisitions				16,025
Translation adjustments			3,228	121
Goodwill, Ending Balance			238,913	101,311
Consulting Segment [Member]
Goodwill [Line Items]
Reassignment	8,143
Translation adjustments			155
Goodwill, Ending Balance			8,298
Product Segment [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance				12,518
Reassignment	$ (12,338)	12,405		(12,518)
Translation adjustments			(67)
Sirius Decisions Segment [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance		142,584	$ 142,584
Reassignment		$ (142,584)
Acquisitions				142,544
Translation adjustments				40
Goodwill, Ending Balance				$ 142,584