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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 606,119	$ 512,098	$ 1,157,393	$ 1,009,872
Adjusted Segment EBITDA	125,395	97,339	242,885	192,620
Corporate Finance
Segment Reporting Information [Line Items]
Revenues	190,003	141,355	350,969	284,277
Adjusted Segment EBITDA	50,492	35,777	87,853	70,581
FLC
Segment Reporting Information [Line Items]
Revenues	145,870	133,527	284,867	261,566
Adjusted Segment EBITDA	28,241	27,615	60,058	53,372
Economic Consulting
Segment Reporting Information [Line Items]
Revenues	155,502	133,308	297,773	266,417
Adjusted Segment EBITDA	23,313	15,472	47,353	34,608
Technology
Segment Reporting Information [Line Items]
Revenues	55,632	46,429	106,968	87,343
Adjusted Segment EBITDA	12,875	7,508	25,598	13,240
Strategic Communications
Segment Reporting Information [Line Items]
Revenues	59,112	57,479	116,816	110,269
Adjusted Segment EBITDA	$ 10,474	$ 10,967	$ 22,023	$ 20,819