Investments of Operating Entities and Consolidated Funds - Operating Entities - Securities Sold, Not Yet Purchased (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 1,361,723,000	$ 728,115,000
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	1,352,771,000	699,894,000
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	40,000	11,358,000
Government Debt Securities
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	0	1,500,000
Preferred Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	6,624,000	6,589,000
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	$ 2,288,000	$ 8,774,000