Segment Information - Segment's Contribution with Respect to Net Revenue, Operating Expenses, Operating Income (Loss) and Total Assets (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]
Net Revenue	$ 687,235	$ 626,481	$ 757,938	$ 754,698	$ 678,808	$ 624,859	$ 717,180	$ 623,464	$ 2,826,352	$ 2,644,311	$ 2,333,371
Operating Expenses	530,918	487,800	557,998	568,870	320,272	480,647	544,679	473,267	2,145,586	1,818,865	1,815,910
Operating Income (Loss)	156,317	$ 138,681	$ 199,940	$ 185,828	358,536	$ 144,212	$ 172,501	$ 150,197	680,766	825,446	517,461
Total Assets	4,997,241				4,928,677				4,997,241	4,928,677	4,556,508
Depreciation and amortization of property									34,103	31,498	33,264
Operating Segments [Member] | Financial Advisory Segment [Member]
Segment Reporting Information [Line Items]
Net Revenue									1,555,526	1,387,682	1,301,044
Operating Expenses									1,198,807	1,143,586	1,017,055
Operating Income (Loss)									356,719	244,096	283,989
Total Assets	859,306				843,142				859,306	843,142	907,035
Depreciation and amortization of property									7,720	6,596	5,661
Operating Segments [Member] | Asset Management Segment [Member]
Segment Reporting Information [Line Items]
Net Revenue									1,331,801	1,255,820	1,051,316
Operating Expenses									912,110	810,870	769,737
Operating Income (Loss)									419,691	444,950	281,579
Total Assets	728,220				756,398				728,220	756,398	645,653
Depreciation and amortization of property									3,253	2,830	3,666
Corporate [Member]
Segment Reporting Information [Line Items]
Net Revenue									(60,975)	809	(18,989)
Operating Expenses									34,669	(135,591)	29,118
Operating Income (Loss)									(95,644)	136,400	(48,107)
Total Assets	$ 3,409,715				$ 3,329,137				3,409,715	3,329,137	3,003,820
Depreciation and amortization of property									$ 23,130	$ 22,072	$ 23,937