Segment Reporting - Revenues and Adjusted Segment Earnings before Interest, Taxes, Depreciation and Amortization for Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 604,593	$ 551,274
Adjusted Segment EBITDA	106,124	117,490
Corporate Finance
Segment Reporting Information [Line Items]
Revenues	207,749	160,966
Adjusted Segment EBITDA	48,946	37,361
FLC
Segment Reporting Information [Line Items]
Revenues	147,597	138,997
Adjusted Segment EBITDA	21,208	31,817
Economic Consulting
Segment Reporting Information [Line Items]
Revenues	132,138	142,271
Adjusted Segment EBITDA	12,710	24,040
Technology
Segment Reporting Information [Line Items]
Revenues	58,723	51,336
Adjusted Segment EBITDA	14,484	12,723
Strategic Communications
Segment Reporting Information [Line Items]
Revenues	58,386	57,704
Adjusted Segment EBITDA	$ 8,776	$ 11,549