Investments of Operating Entities and Consolidated Funds - Operating Entities - Securities Sold, Not Yet Purchased (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 931,082,000	$ 728,115,000
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	918,021,000	699,894,000
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	453,000	11,358,000
US Government Debt Securities
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	522,000	1,500,000
Preferred Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	7,567,000	6,589,000
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 4,519,000	$ 8,774,000