Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value - (Details) - USD ($)		6 Months Ended	12 Months Ended
		Jun. 30, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Fair Value, Concentration of Risk, Investments		$ 0	$ 0
Marketable Securities, Consolidated Funds		53,528,000	32,000,000
Securities owned, at fair value		471,754,000	610,234,000
Securities sold, not yet purchased, at fair value		285,414,000	257,159,000
US Government Securities
Investment Holdings [Line Items]
Trading Securities, Debt	[1]	$ 3,773,000	$ 3,016,000
Debt securities, interest rate		0.00%
US Government Securities | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate			0.00%
US Government Securities | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate			5.95%
Preferred Stock
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 41,148,000	$ 32,000,000
Trading Securities, Equity	[2]	13,242,000	25,563,000
Common Stock
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		6,228,000	0
Trading Securities, Equity	[2]	443,240,000	516,108,000
Securities sold, not yet purchased, at fair value		284,283,000	257,101,000
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[3]	$ 250,000	$ 819,000
Debt securities, interest rate		8.00%
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]
Marketable Securities, Consolidated Funds	[4]	$ 4,726,000	$ 0
Trading Securities, Debt	[5]	6,847,000	47,192,000
Securities sold, not yet purchased, at fair value	[6]	$ 1,131,000	$ 58,000
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate		3.50%	3.25%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate		8.25%	9.00%
Warrants and Rights
Investment Holdings [Line Items]
Trading Securities, Equity		$ 3,581,000	$ 3,059,000
Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity	[7]	821,000	14,477,000
Loans
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 1,426,000	0
Consolidated Funds | Corporate Bonds | Minimum
Investment Holdings [Line Items]
Debt securities, interest rate		6.28%
Consolidated Funds | Corporate Bonds | Maximum
Investment Holdings [Line Items]
Debt securities, interest rate		14.37%
Enterprise Master
Investment Holdings [Line Items]
Securities owned, at fair value		$ 2,382,000	2,781,000
Enterprise Master | Restricted Stock
Investment Holdings [Line Items]
Securities owned, at fair value		124,000	124,000
Enterprise Master | Preferred Stock
Investment Holdings [Line Items]
Securities owned, at fair value		1,493,000	1,484,000
Enterprise Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		765,000	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		499,076,000	157,921,000
Merger Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		499,076,000	157,429,000
Securities sold, not yet purchased, at fair value		325,100,000	73,800,000
Merger Master | Corporate Bonds
Investment Holdings [Line Items]
Securities owned, at fair value		0	$ 492,000	[8]
Debt securities, interest rate			5.25%
Caerus Master [Member] | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		4,800,000
Securities sold, not yet purchased, at fair value		4,800,000
Common Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[2]		$ 7,400,000
Preferred Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[2]	$ 4,800,000	7,700,000
Affiliated Entity | Mutual Funds
Investment Holdings [Line Items]
Trading Securities, Equity	[7]		$ 13,400,000

[1] As of June 30, 2016, maturities ranged from July 2016 to June 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%.

[2] Included in preferred stocks are investments in securities for which the Company has elected the fair value option with the fair value of $4.8 million at June 30, 2016 and preferred and common stock of $7.7 million and $7.4 million, respectively, at December 31, 2015. These investments were acquired in connection with merchant banking transactions.

[3] As of June 30, 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%.

[4] As of June 30, 2016, maturities ranged from October 2017 to May 2049 and interest rates ranged between 6.28% and 14.37%.

[5] As of June 30, 2016, maturities ranged from August 2017 to February 2046 and interest rates ranged between 3.50% to 8.25%. As of December 31, 2015, maturities ranged from March 2016 to February 2046 and interest rates ranged between 3.25% to 9.00%.

[6] As of June 30, 2016 and December 31, 2015, the maturities ranged from February 2020 to January 2026 with interest rates ranged between 5.55%

[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%.