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, 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]
Net Revenue	$ 678,808	$ 624,859	$ 717,180	$ 623,464	$ 691,565	$ 608,908	$ 534,680	$ 498,218	$ 2,644,311	$ 2,333,371	$ 2,353,608
Operating Expenses	320,272	480,647	544,679	473,267	534,709	459,916	421,444	399,841	1,818,865	1,815,910	2,370,228
Operating Income (Loss)	358,536	$ 144,212	$ 172,501	$ 150,197	156,856	$ 148,992	$ 113,236	$ 98,377	825,446	517,461	(16,620)
Total Assets	4,928,677				4,556,508				4,928,677	4,556,508	4,477,774
Depreciation and amortization of property									31,498	33,264	32,785
Operating Segments [Member] | Financial Advisory Segment [Member]
Segment Reporting Information [Line Items]
Net Revenue									1,387,682	1,301,044	1,279,628
Operating Expenses									1,143,586	1,017,055	1,005,837
Operating Income (Loss)									244,096	283,989	273,791
Total Assets	843,142				907,035				843,142	907,035	763,374
Depreciation and amortization of property									6,596	5,661	4,412
Operating Segments [Member] | Asset Management Segment [Member]
Segment Reporting Information [Line Items]
Net Revenue									1,255,820	1,051,316	1,111,105
Operating Expenses									810,870	769,737	736,798
Operating Income (Loss)									444,950	281,579	374,307
Total Assets	756,398				645,653				756,398	645,653	640,034
Depreciation and amortization of property									2,830	3,666	2,957
Corporate [Member]
Segment Reporting Information [Line Items]
Net Revenue									809	(18,989)	(37,125)
Operating Expenses									(135,591)	29,118	627,593
Operating Income (Loss)									136,400	(48,107)	(664,718)
Total Assets	$ 3,329,137				$ 3,003,820				3,329,137	3,003,820	3,074,366
Depreciation and amortization of property									$ 22,072	$ 23,937	$ 25,416