Investments of Operating Entities and Consolidated Funds - Merger Master Narrative (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Trading securities		$ 922,164	$ 520,888
Securities sold, not yet purchased, at fair value		364,742	195,307
Merger Master
Investment [Line Items]
Trading securities		177,012	279,299
Securities sold, not yet purchased, at fair value		58,444	9,610
Common Stock
Investment [Line Items]
Securities sold, not yet purchased, at fair value		349,408	194,305
Common Stock | Merger Master
Investment [Line Items]
Trading securities		166,669	162,811
Securities sold, not yet purchased, at fair value		58,444	9,610
Corporate Bonds
Investment [Line Items]
Securities sold, not yet purchased, at fair value	[1]	1,281	750
Corporate Bonds | Merger Master
Investment [Line Items]
Trading securities		$ 10,343	$ 116,488

[1] As of September 30, 2019, the maturities ranged from February 2024 to April 2037 and interest rates ranged from 5.00% to 5.50%. As of December 31, 2018, the maturities ranged from October 2022 to January 2034 and interest rates ranged from 2.25% to 9.38%.