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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 593,106	$ 513,012	$ 1,750,499	$ 1,522,884
Adjusted Segment EBITDA	119,435	94,281	362,320	286,901
Corporate Finance
Segment Reporting Information [Line Items]
Revenues	191,698	135,418	542,667	419,695
Adjusted Segment EBITDA	48,084	26,798	135,937	97,379
FLC
Segment Reporting Information [Line Items]
Revenues	142,651	126,684	427,518	388,250
Adjusted Segment EBITDA	27,008	21,970	87,066	75,342
Economic Consulting
Segment Reporting Information [Line Items]
Revenues	141,715	139,166	439,488	405,583
Adjusted Segment EBITDA	19,413	23,238	66,766	57,846
Technology
Segment Reporting Information [Line Items]
Revenues	57,083	56,692	164,051	144,035
Adjusted Segment EBITDA	12,286	11,473	37,884	24,713
Strategic Communications
Segment Reporting Information [Line Items]
Revenues	59,959	55,052	176,775	165,321
Adjusted Segment EBITDA	$ 12,644	$ 10,802	$ 34,667	$ 31,621