Investments of Operating Entities and Consolidated Funds - Consolidated Funds - Securities Owned and Sold By Merger Master (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Securities owned, at fair value ($254,383 and $941,595 were pledged to various parties)	$ 1,088,566,000	$ 1,633,552,000
Securities sold, not yet purchased, at fair value	440,930,000	451,836,000
Portfolio Funds, Consolidated Funds	98,267,000	175,769,000
Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value ($254,383 and $941,595 were pledged to various parties)		79,353,000
Securities sold, not yet purchased, at fair value		68,150,000
Merger Fund
Investment Holdings [Line Items]
Portfolio Funds, Consolidated Funds	0	76,616,000
Common Stock
Investment Holdings [Line Items]
Securities sold, not yet purchased, at fair value	404,409,000	425,448,000
Common Stock | Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value ($254,383 and $941,595 were pledged to various parties)		76,531,000
Securities sold, not yet purchased, at fair value		29,623,000
Warrants and Rights [Member]
Investment Holdings [Line Items]
Securities sold, not yet purchased, at fair value	8,930,000	14,819,000
Warrants and Rights [Member] | Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value ($254,383 and $941,595 were pledged to various parties)		748,000
Corporate Bonds
Investment Holdings [Line Items]
Securities sold, not yet purchased, at fair value	$ 15,324,000	5,933,000
Corporate Bonds | Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value ($254,383 and $941,595 were pledged to various parties)		2,074,000
Exchange Traded Funds | Merger Master
Investment Holdings [Line Items]
Securities sold, not yet purchased, at fair value		$ 38,527,000