Segment Reporting - Revenues and Adjusted Segment Earnings before Interest, Taxes, Depreciation and Amortization for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 497,774	$ 446,344
Adjusted Segment EBITDA	95,281	56,017
Corporate Finance & Restructuring
Segment Reporting Information [Line Items]
Revenues	142,922	105,901
Adjusted Segment EBITDA	34,804	10,325
Forensic and Litigation Consulting
Segment Reporting Information [Line Items]
Revenues	128,039	111,406
Adjusted Segment EBITDA	25,757	13,521
Economic Consulting
Segment Reporting Information [Line Items]
Revenues	133,109	139,221
Adjusted Segment EBITDA	19,136	20,110
Technology
Segment Reporting Information [Line Items]
Revenues	40,914	46,087
Adjusted Segment EBITDA	5,732	7,804
Strategic Communications
Segment Reporting Information [Line Items]
Revenues	52,790	43,729
Adjusted Segment EBITDA	$ 9,852	$ 4,257