Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value - Quarterly (Details) - USD ($)		9 Months Ended	12 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Fair Value, Concentration of Risk, Investments		$ 0	$ 0
Marketable Securities, Consolidated Funds		19,000,000	0
Securities owned, at fair value		$ 864,685,000	$ 792,206,000
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		$ 15,771,000	$ 15,070,000
Common Stock
Investment Holdings [Line Items]
Trading Securities, Equity		712,489,000	597,476,000
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[1]	$ 1,129,000	$ 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	[2]	$ 113,585,000	$ 159,557,000
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate		3.17%	5.63%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate		21.74%	11.54%
US Treasury and Government
Investment Holdings [Line Items]
Trading Securities, Debt	[3]	$ 3,015,000	$ 2,010,000
Warrants and Rights
Investment Holdings [Line Items]
Trading Securities, Equity		3,871,000	1,417,000
Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity		14,825,000	15,776,000
Enterprise Master
Investment Holdings [Line Items]
Securities owned, at fair value		5,250,000	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		626,000	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		1,657,000	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		2,715,000	2,802,000
Merger Master
Investment Holdings [Line Items]
Securities owned, at fair value		179,650,000	136,893,000
Merger Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		179,650,000	133,510,000
Merger Master | Corporate Bonds
Investment Holdings [Line Items]
Securities owned, at fair value	[4]	0	$ 3,383,000
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%
Quadratic Master
Investment Holdings [Line Items]
Securities owned, at fair value		15,267,000
Quadratic Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		267,000
Quadratic Master | US Treasury and Government
Investment Holdings [Line Items]
Securities owned, at fair value	[5]	$ 15,000,000
Debt securities, interest rate		0.00%

[1] As of September 30, 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%.

[2] As of September 30, 2015, maturities ranged from March 2016 to February 2046 and interest rates ranged between 3.17% to 21.74%. As of December 31, 2014, maturities ranged from February 2015 to February 2046 and interest rates ranged between 5.63% to 11.54%.

[3] As of September 30, 2015, maturities ranged from December 2015 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%.

[4] As of December 31, 2014, maturities ranged from February 2017 to June 2019 and interest rates ranged between 8.50% and 9.75%.

[5] As of September 30, 2015, the maturity was October 2015 with an interest rate of 0%.