Investments of Operating Entities and Consolidated Funds - Consolidated Funds - Securities Owned and Sold By Merger Master (Details) - USD ($)		Mar. 31, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Securities owned, at fair value ($200,278 and $941,595 were pledged to various parties)		$ 776,046,000	$ 1,633,552,000
Securities sold, not yet purchased, at fair value		527,471,000	451,836,000
Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value ($200,278 and $941,595 were pledged to various parties)		179,326,000	79,353,000
Securities sold, not yet purchased, at fair value		37,082,000	68,150,000
Common Stock
Investment Holdings [Line Items]
Securities sold, not yet purchased, at fair value		460,059,000	425,448,000
Common Stock | Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value ($200,278 and $941,595 were pledged to various parties)		175,892,000	76,531,000
Securities sold, not yet purchased, at fair value		30,184,000	29,623,000
Warrants and Rights [Member]
Investment Holdings [Line Items]
Securities sold, not yet purchased, at fair value		8,532,000	14,819,000
Warrants and Rights [Member] | Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value ($200,278 and $941,595 were pledged to various parties)		1,365,000	748,000
Corporate Bonds
Investment Holdings [Line Items]
Securities sold, not yet purchased, at fair value	[1]	4,402,000	5,933,000
Corporate Bonds | Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value ($200,278 and $941,595 were pledged to various parties)		2,069,000	2,074,000
Exchange Traded Funds | Merger Master
Investment Holdings [Line Items]
Securities sold, not yet purchased, at fair value		$ 6,898,000	$ 38,527,000

[1] As of March 31, 2020, the maturities ranged from April 2020 to May 2067 and interest rates ranged from 5.00% to 6.75%. As of December 31, 2019, the maturities ranged from January 2024 to May 2037 and interest rates ranged from 4.88% to 6.25%.