Investments of Operating Entities and Consolidated Funds - Securities Sold, Not Yet Purchased (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, Consolidated Funds		$ 883,000	$ 0
Securities sold, not yet purchased, at fair value		$ 266,090,000	$ 257,159,000
Minimum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		5.50%
Maximum
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, interest rate		6.25%
Convertible Debt Securities [Member]
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		8.00%
Convertible Debt Securities [Member] | Minimum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate			8.00%
Convertible Debt Securities [Member] | Maximum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate			10.00%
Corporate Bonds
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, Consolidated Funds	[1]	$ 672,000
Securities sold, not yet purchased, at fair value	[2]	$ 2,591,000	$ 58,000
Corporate Bonds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		6.25%	3.25%
Securities sold, not yet purchased, interest rate		5.50%	5.50%
Corporate Bonds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		13.00%	9.00%
Securities sold, not yet purchased, interest rate		6.25%	6.25%
Corporate Bonds | Consolidated Funds | Minimum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		0.00%
Securities sold, not yet purchased, interest rate		4.38%
Corporate Bonds | Consolidated Funds | Maximum
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate		14.37%
Securities sold, not yet purchased, interest rate		9.25%
Corporate Bonds | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Investment Interest Rate			5.25%
Common Stock
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, Consolidated Funds		$ 211,000
Securities sold, not yet purchased, at fair value		263,460,000	$ 257,101,000
Common Stock | Merger Master
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		395,500,000	73,800,000
Common Stock | Caerus Master [Member]
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		2,600,000
Warrants and Rights
Investments Sold, Not yet Purchased [Line Items]
Securities sold, not yet purchased, at fair value		$ 39,000	$ 0

[1] As of December 31, 2016, maturities ranged from September 2019 to September 2023 and interest rates ranged between 4.38% and 9.25%

[2] As of December 31, 2016 and 2015, the maturities ranged from April 2021 to January 2036 with interest rates ranged between 5.50% to 6.25%