Segment Reporting - Revenues and Adjusted Segment Earnings before Interest, Taxes, Depreciation and Amortization for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 467,711	$ 448,962	$ 444,715	$ 446,344	$ 441,920	$ 438,042	$ 460,147	$ 470,285	$ 1,807,732	$ 1,810,394	$ 1,779,149
Adjusted Segment EBITDA									267,435	285,944	283,435
Corporate Finance & Restructuring
Segment Reporting Information [Line Items]
Revenues									482,041	483,269	440,398
Adjusted Segment EBITDA									82,863	97,688	90,101
Forensic and Litigation Consulting
Segment Reporting Information [Line Items]
Revenues									462,324	457,734	482,269
Adjusted Segment EBITDA									72,705	57,882	64,267
Economic Consulting
Segment Reporting Information [Line Items]
Revenues									496,029	500,487	447,909
Adjusted Segment EBITDA									61,964	74,102	62,330
Technology
Segment Reporting Information [Line Items]
Revenues									174,850	177,720	218,599
Adjusted Segment EBITDA									22,171	25,814	39,010
Strategic Communications
Segment Reporting Information [Line Items]
Revenues									192,488	191,184	189,974
Adjusted Segment EBITDA									$ 27,732	$ 30,458	$ 27,727