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Goodwill and Other Intangible Assets - Schedule of Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Thousands			12 Months Ended
		Jan. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Balance as of January 1			$ 719,934	$ 715,644
Effect of foreign currency translation			1,726	1,663
Balance as of December 31			909,913	719,934
ITG
Goodwill [Line Items]
Goodwill resulting from business combination		$ 188,300	$ 188,253
DMS Disaster Consultants
Goodwill [Line Items]
Goodwill resulting from business combination				(50)
Vista Limited
Goodwill [Line Items]
Goodwill resulting from business combination				(370)
Olson
Goodwill [Line Items]
Goodwill resulting from business combination	[1]			$ 3,047
[1] In 2019, the Company recorded changes to goodwill representing an immaterial correction of an error for income tax balances related to acquired assets and liabilities from the business combination that occurred in 2014.  These balances were not significant to our previously reported financial position.

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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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