Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($)		Mar. 31, 2018	Dec. 31, 2017
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 261,586,000	$ 342,527,000
Securities sold, not yet purchased, at fair value, Consolidated Funds		0
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		252,585,000	342,328,000
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[1]	$ 1,287,000	$ 122,000
Corporate Bonds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		5.55%	3.78%
Corporate Bonds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		7.50%	5.55%
Preferred Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 450,000	$ 77,000
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 7,264,000	$ 0

[1] As of March 31, 2018, the maturities ranged from August 2018 to January 2026 with interest rates ranged from 5.55% to 7.50%. As of December 31, 2017, the maturities ranged from July 2025 to January 2026 with interest rates ranged from 3.78% to 5.55%.