Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($)		Sep. 30, 2018	Dec. 31, 2017
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 374,518,000	$ 342,527,000
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		353,113,000	342,328,000
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[1]	$ 20,309,000	$ 122,000
Corporate Bonds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		5.00%	3.78%
Corporate Bonds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		5.55%	5.55%
Preferred Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 836,000	$ 77,000
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 260,000	$ 0

[1] As of September 30, 2018, the maturities ranged from January 2026 to April 2049 with interest rates ranged from 5.00% to 5.55%. As of December 31, 2017, the maturities ranged from July 2025 to January 2026 with interest rates ranged from 3.78% to 5.55%.