Segment Reporting - Revenues and Adjusted Segment Earnings before Interest, Taxes, Depreciation and Amortization for Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 607,852	$ 606,119	$ 1,212,445	$ 1,157,393
Adjusted Segment EBITDA	105,380	125,395	211,504	242,885
Corporate Finance
Segment Reporting Information [Line Items]
Revenues	246,011	190,003	453,760	350,969
Adjusted Segment EBITDA	76,264	50,492	125,210	87,853
FLC
Segment Reporting Information [Line Items]
Revenues	106,381	145,870	253,978	284,867
Adjusted Segment EBITDA	(9,047)	28,241	12,161	60,058
Economic Consulting
Segment Reporting Information [Line Items]
Revenues	151,493	155,502	283,631	297,773
Adjusted Segment EBITDA	21,694	23,313	34,404	47,353
Technology
Segment Reporting Information [Line Items]
Revenues	47,084	55,632	105,807	106,968
Adjusted Segment EBITDA	6,435	12,875	20,919	25,598
Strategic Communications
Segment Reporting Information [Line Items]
Revenues	56,883	59,112	115,269	116,816
Adjusted Segment EBITDA	$ 10,034	$ 10,474	$ 18,810	$ 22,023