Segment Reporting - Revenues and Adjusted Segment Earnings before Interest, Taxes, Depreciation and Amortization for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 512,098	$ 444,715	$ 1,009,872	$ 891,059
Adjusted Segment EBITDA	97,339	58,886	192,620	114,903
Corporate Finance & Restructuring
Segment Reporting Information [Line Items]
Revenues	141,355	117,487	284,277	223,388
Adjusted Segment EBITDA	35,777	20,048	70,581	30,373
Forensic and Litigation Consulting
Segment Reporting Information [Line Items]
Revenues	133,527	111,410	261,566	222,816
Adjusted Segment EBITDA	27,615	13,032	53,372	26,553
Economic Consulting
Segment Reporting Information [Line Items]
Revenues	133,308	124,004	266,417	263,225
Adjusted Segment EBITDA	15,472	15,509	34,608	35,619
Technology
Segment Reporting Information [Line Items]
Revenues	46,429	45,566	87,343	91,653
Adjusted Segment EBITDA	7,508	5,421	13,240	13,225
Strategic Communications
Segment Reporting Information [Line Items]
Revenues	57,479	46,248	110,269	89,977
Adjusted Segment EBITDA	$ 10,967	$ 4,876	$ 20,819	$ 9,133