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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 504,993	$ 513,012	$ 512,098	$ 497,774	$ 467,711	$ 448,962	$ 444,715	$ 446,344	$ 2,027,877	$ 1,807,732	$ 1,810,394
Adjusted Segment EBITDA									358,741	267,435	285,944
Corporate Finance
Segment Reporting Information [Line Items]
Revenues									564,479	482,041	483,269
Adjusted Segment EBITDA									121,660	82,863	97,688
FLC
Segment Reporting Information [Line Items]
Revenues									520,333	462,324	457,734
Adjusted Segment EBITDA									96,821	72,705	57,882
Economic Consulting
Segment Reporting Information [Line Items]
Revenues									533,979	496,029	500,487
Adjusted Segment EBITDA									69,955	61,964	74,102
Technology
Segment Reporting Information [Line Items]
Revenues									185,755	174,850	177,720
Adjusted Segment EBITDA									27,387	22,171	25,814
Strategic Communications
Segment Reporting Information [Line Items]
Revenues									223,331	192,488	191,184
Adjusted Segment EBITDA									$ 42,918	$ 27,732	$ 30,458