Goodwill and Other Intangible Assets - Schedule of Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Balance as of January 1		$ 715,644	$ 686,108
Effect of foreign currency translation		1,663	(2,574)
Balance as of December 31		719,934	715,644
The Future Customer
Goodwill [Line Items]
Goodwill resulting from business combinations			7,597
DMS Disaster Consultants
Goodwill [Line Items]
Goodwill resulting from business combinations		(50)	10,121
Vista Limited
Goodwill [Line Items]
Goodwill resulting from business combinations		(370)	$ 14,392
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.