Investments of Operating Entities and Consolidated Funds - Operating Entities - Securities Sold, Not Yet Purchased (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 440,930,000	$ 451,836,000
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	404,409,000	425,448,000
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	15,324,000	5,933,000
US Government Debt Securities
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	1,500,000	1,950,000
Preferred Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	10,767,000	3,686,000
Warrants and Rights [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 8,930,000	$ 14,819,000