Segment Reporting - Revenues and Adjusted Segment Earnings before Interest, Taxes, Depreciation and Amortization for Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 622,249	$ 593,106	$ 1,834,694	$ 1,750,499
Adjusted Segment EBITDA	115,892	119,435	327,396	362,320
Corporate Finance
Segment Reporting Information [Line Items]
Revenues	236,615	191,698	690,375	542,667
Adjusted Segment EBITDA	56,215	48,084	181,425	135,937
FLC
Segment Reporting Information [Line Items]
Revenues	119,104	142,651	373,082	427,518
Adjusted Segment EBITDA	13,591	27,008	25,752	87,066
Economic Consulting
Segment Reporting Information [Line Items]
Revenues	154,978	141,715	438,609	439,488
Adjusted Segment EBITDA	25,720	19,413	60,124	66,766
Technology
Segment Reporting Information [Line Items]
Revenues	58,585	57,083	164,392	164,051
Adjusted Segment EBITDA	11,939	12,286	32,858	37,884
Strategic Communications
Segment Reporting Information [Line Items]
Revenues	52,967	59,959	168,236	176,775
Adjusted Segment EBITDA	$ 8,427	$ 12,644	$ 27,237	$ 34,667