Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value - (Details) - USD ($)		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Fair Value, Concentration of Risk, Investments		$ 0	$ 0
Marketable Securities, Consolidated Funds		32,000,000	0
Securities owned, at fair value		610,234,000	792,206,000
US Treasury and Government [Member]
Investment Holdings [Line Items]
Trading Securities, Debt	[1]	$ 3,016,000	$ 2,010,000
US Government Securities | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate		0.00%	0.00%
US Government Securities | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate		5.95%	5.95%
Preferred Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[2]	$ 25,563,000	$ 15,070,000
Common Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[2]	516,108,000	597,476,000
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[3]	$ 819,000	$ 900,000
Debt securities, interest rate			10.00%
Convertible Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate		8.00%
Convertible Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate		10.00%
Corporate Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[4]	$ 47,192,000	$ 159,557,000
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate		3.25%	5.63%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate		9.00%	11.54%
Warrants and Rights
Investment Holdings [Line Items]
Trading Securities, Equity		$ 3,059,000	$ 1,417,000
Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity	[5]	14,477,000	15,776,000
Enterprise Master
Investment Holdings [Line Items]
Securities owned, at fair value		2,781,000	5,896,000
Enterprise Master | Debt
Investment Holdings [Line Items]
Securities owned, at fair value		0	20,000
Enterprise Master | Trade Claims
Investment Holdings [Line Items]
Securities owned, at fair value		128,000	128,000
Enterprise Master | Preferred Stock
Investment Holdings [Line Items]
Securities owned, at fair value		1,484,000	576,000
Enterprise Master | Private Equity
Investment Holdings [Line Items]
Securities owned, at fair value		0	587,000
Enterprise Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		724,000	1,659,000
Enterprise Master | Restricted Stock
Investment Holdings [Line Items]
Securities owned, at fair value		124,000	124,000
Enterprise Master | Rights
Investment Holdings [Line Items]
Securities owned, at fair value		321,000	2,802,000
Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value		157,921,000	136,893,000
Merger Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		157,429,000	133,510,000
Merger Master | Corporate Bonds
Investment Holdings [Line Items]
Securities owned, at fair value	[6]	$ 492,000	$ 3,383,000
Debt securities, interest rate		5.25%
Merger Master | Corporate Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate			8.50%
Merger Master | Corporate Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate			9.75%
Common Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[2]	$ 7,400,000	$ 14,300,000
Preferred Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[2]	7,700,000
Affiliated Entity | Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity	[5]	$ 13,400,000	$ 15,700,000

[1] As of December 31, 2015, maturities ranged from January 2016 to August 2016 with interest rates ranged between 0% to 5.95%. As of December 31, 2014, maturities ranged from May 2015 to April 2016 with interest rates ranged between 0% to 5.95%.

[2] Included in preferred stocks and common stocks are investments in securities for which the Company has elected the fair value option with the fair value of $7.7 million and $7.4 million, respectively, at December 31, 2015 and $14.3 million of common stocks at December 31, 2014. These investments were acquired in contemplation with merchant banking transactions.

[3] As of December 31, 2015, maturities ranged from July 2016 to March 2018 with interest rates ranged between 8% to 10.00%. As of December 31, 2014, the maturity was February 2015 with an interest rate of 10.00%.

[4] As of December 31, 2015, maturities ranged from March 2016 to February 2046 and interest rates ranged between 3.25% to 9.00%. As of December 31, 2014, maturities ranged from February 2015 to February 2046 and interest rates ranged between 5.63% to 11.54%.

[5] s of December 31, 2015 and 2014, are investments in affiliated funds of $13.4 million and $15.7 million, respectively.

[6] As of December 31, 2015, the maturity was ranged from June 2024 with interest rate of 5.25%. As of December 31, 2014, maturities ranged from February 2017 to June 2019 and interest rates ranged between 8.50% and 9.75%.