Investments of Operating Entities and Consolidated Funds - Merger Master Narrative (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Trading securities		$ 900,346	$ 520,888
Securities sold, not yet purchased, at fair value		438,584	195,307
Merger Master
Investment [Line Items]
Trading securities		394,936	279,299
Securities sold, not yet purchased, at fair value		111,074	9,610
Common Stock
Investment [Line Items]
Securities sold, not yet purchased, at fair value		409,034	194,305
Common Stock | Merger Master
Investment [Line Items]
Trading securities		324,742	162,811
Securities sold, not yet purchased, at fair value		111,074	9,610
Corporate Bonds
Investment [Line Items]
Securities sold, not yet purchased, at fair value	[1]	2,581	750
Corporate Bonds | Merger Master
Investment [Line Items]
Trading securities		$ 70,194	$ 116,488

[1] As of June 30, 2019, the maturities ranged from October 2020 to May 2029 and interest rates ranged from 1.88% to 15.50%. As of December 31, 2018, the maturities ranged from October 2022 to January 2034 and interest rates ranged from 2.25% to 9.38%.