Investments of Operating Entities and Consolidated Funds - Merger Master Narrative (Details) - USD ($)		Dec. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Trading securities		$ 1,633,552,000	$ 520,888,000
Securities sold, not yet purchased, at fair value		451,836,000	195,307,000
Merger Master
Investment [Line Items]
Trading securities		79,353,000	279,299,000
Securities sold, not yet purchased, at fair value		68,150,000	9,610,000
Common Stock
Investment [Line Items]
Securities sold, not yet purchased, at fair value		425,448,000	194,305,000
Common Stock | Merger Master
Investment [Line Items]
Trading securities		76,531,000	162,811,000
Securities sold, not yet purchased, at fair value		29,623,000	4,959,000
Corporate Bonds
Investment [Line Items]
Securities sold, not yet purchased, at fair value	[1]	5,933,000	750,000
Corporate Bonds | Merger Master
Investment [Line Items]
Trading securities		2,074,000	116,488,000
Exchange Traded Funds | Merger Master
Investment [Line Items]
Securities sold, not yet purchased, at fair value		38,527,000	4,651,000
Warrants and Rights [Member]
Investment [Line Items]
Securities sold, not yet purchased, at fair value		14,819,000	53,000
Warrants and Rights [Member] | Merger Master
Investment [Line Items]
Trading securities		$ 748,000	$ 0

[1] As of December 31, 2019, the maturities ranged from January 2024 to May 2037 and interest rates ranged from 4.88% to 6.25%. As of December 31, 2018, the maturities ranged from October 2022 to January 2034 and interest rates ranged from 2.25% to 9.38%.