Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value - (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015
Investment Holdings [Line Items]
Fair Value, Concentration of Risk, Investments		$ 0		$ 0
Marketable Securities, Consolidated Funds		79,237,000		32,000,000
Securities owned, at fair value		700,876,000		609,234,000
US Government Securities
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		6,994,000	[1]	0
Trading Securities, Debt	[2],[3]	$ 3,780,000		$ 3,016,000
Investment Interest Rate		0.00%
US Government Securities | Minimum
Investment Holdings [Line Items]
Investment Interest Rate				0.00%
US Government Securities | Maximum
Investment Holdings [Line Items]
Investment Interest Rate				5.95%
Preferred Stock
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 37,343,000		$ 32,000,000
Trading Securities, Equity	[3]	15,811,000		25,563,000
Common Stock
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		28,474,000		0
Trading Securities, Equity	[3]	669,655,000		515,108,000
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[4]	$ 250,000		$ 819,000
Investment Interest Rate		8.00%
Convertible Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate				8.00%
Convertible Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate				10.00%
Corporate Bonds
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 4,214,000	[5]	$ 0
Trading Securities, Debt	[6]	$ 3,029,000		$ 47,192,000
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		6.25%		3.25%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		13.00%		9.00%
Loans
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 2,209,000	[5]	$ 0
Warrants and Rights
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		3,000		0
Trading Securities, Equity		8,335,000		3,059,000
Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity	[3],[7]	6,000		14,477,000
Trade Claims
Investment Holdings [Line Items]
Trading Securities, Debt		$ 10,000		0
Consolidated Funds | US Government Securities
Investment Holdings [Line Items]
Investment Interest Rate		0.00%
Consolidated Funds | Corporate Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		0.00%
Consolidated Funds | Corporate Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		14.37%
Enterprise Master
Investment Holdings [Line Items]
Securities owned, at fair value		$ 2,416,000		2,781,000
Enterprise Master | Restricted Stock
Investment Holdings [Line Items]
Securities owned, at fair value		0		124,000
Enterprise Master | Preferred Stock
Investment Holdings [Line Items]
Securities owned, at fair value		1,581,000		1,484,000
Enterprise Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		835,000		724,000
Enterprise Master | Rights
Investment Holdings [Line Items]
Securities owned, at fair value		0		321,000
Enterprise Master | Trade Claims
Investment Holdings [Line Items]
Securities owned, at fair value		0		128,000
Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value		835,672,000		157,921,000
Merger Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		835,672,000		157,429,000
Merger Master | Corporate Bonds
Investment Holdings [Line Items]
Securities owned, at fair value		0		$ 492,000	[8]
Investment Interest Rate				5.25%
Caerus Master [Member] | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		3,200,000
US Government Securities | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Debt	[3]	3,800,000		$ 3,000,000
Mutual Funds | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[3]	100,000		5,500,000
Common Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[3]	5,200,000		9,400,000
Preferred Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[3]	$ 7,000,000		11,600,000
Affiliated Entity | Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity	[7]			$ 13,400,000

[1] As of December 31, 2016, the maturity was March 2017 with an interest rate of 0%.

[2] As of December 31, 2016, maturities ranged from February 2017 to December 2017 with an interest rate of 0%. As of December 31, 2015, maturities ranged from January 2016 to August 2016 with interest rates ranged between 0% to 5.95%.

[3] The Company has elected the fair value option for investments in securities of preferred and common stocks with a fair value of $7.0 million and $5.2 million at December 31, 2016 and preferred and common stock with a fair value of $11.6 million and $9.4 million, respectively, at December 31, 2015. The Company has also elected the fair value option for investments in mutual funds and U.S. government securities with a fair value of $0.1 million and $3.8 million at December 31, 2016 and mutual funds and U.S. government securities with a fair value of $5.5 million and $3.0 million, respectively, at December 31, 2015, respectively.

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

[5] As of December 31, 2016, maturities ranged from October 2017 to June 2038 and interest rates ranged between 0% and 14.37%.

[6] As of December 31, 2016, maturities ranged from January 2017 to January 2036 and interest rates ranged between 6.25% to 13.00%. As of December 31, 2015, maturities ranged from March 2016 to February 2046 and interest rates ranged between 3.25% to 9.00%.

[7] Included in this amount as of December 31, 2015, are investments in affiliated funds of $13.4 million all of which was liquidated during the three months ended March 31, 2016.

[8] As of December 31, 2015, the maturity was June 2024 with an interest rate of 5.25%.