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
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net Revenue	$ 754,698	$ 623,464
Operating Expenses	568,870	473,267
Operating Income (Loss)	185,828	150,197
Total Assets	4,485,095		$ 4,928,677
Financial Advisory Segment [Member]
Segment Reporting Information [Line Items]
Net Revenue	409,121
Asset Management Segment [Member]
Segment Reporting Information [Line Items]
Net Revenue	354,283
Operating Segments [Member] | Financial Advisory Segment [Member]
Segment Reporting Information [Line Items]
Net Revenue	409,121	335,812
Operating Expenses	324,696	262,751
Operating Income (Loss)	84,425	73,061
Total Assets	866,734		843,142
Operating Segments [Member] | Asset Management Segment [Member]
Segment Reporting Information [Line Items]
Net Revenue	354,283	283,363
Operating Expenses	229,669	188,751
Operating Income (Loss)	124,614	94,612
Total Assets	686,724		756,398
Corporate [Member]
Segment Reporting Information [Line Items]
Net Revenue	(8,706)	4,289
Operating Expenses	14,505	21,765
Operating Income (Loss)	(23,211)	$ (17,476)
Total Assets	$ 2,931,637		$ 3,329,137