Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($)		Sep. 30, 2016	Dec. 31, 2015
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 1,540,000	$ 0
Securities sold, not yet purchased, at fair value		212,183,000	257,159,000
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value	[1]	1,145,000
Securities sold, not yet purchased, at fair value	[2]	$ 3,653,000	$ 58,000
Securities sold, not yet purchased, interest rate		5.55%	5.55%
Corporate Bonds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		3.25%	3.25%
Corporate Bonds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		6.88%	6.88%
Corporate Bonds | Consolidated Funds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		4.38%
Corporate Bonds | Consolidated Funds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		9.25%
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 395,000
Securities sold, not yet purchased, at fair value		208,491,000	$ 257,101,000
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		203,800,000	73,800,000
Common Stock | Caerus Master [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		3,800,000
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 39,000	$ 0

[1] As of September 30, 2016, maturities ranged from September 2019 to March 2025 and interest rates ranged between 4.38% and 9.25%.

[2] As of September 30, 2016 and December 31, 2015, the maturities ranged from June 2017 to January 2026 with interest rates ranged between 3.25% to 6.88%