Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value - (Details)		9 Months Ended		12 Months Ended
		Sep. 30, 2017 USD ($) investments		Dec. 31, 2016 USD ($) investments
Investment Holdings [Line Items]
Concentration of Risk, Number of Investments | investments		1		0
Marketable Securities, Consolidated Funds		$ 117,630,000		$ 79,237,000
Securities owned, at fair value		659,772,000		700,876,000
Fair Value, Concentration of Risk, Investments				0
US Government Securities
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds	[1]	21,968,000		6,994,000
Trading Securities, Debt	[2]	$ 2,793,000		$ 3,780,000
Investment Interest Rate		0.00%		0.00%
Preferred Stock
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 18,943,000		$ 37,343,000
Trading Securities, Equity	[3]	17,989,000		15,811,000
Common Stock
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		73,341,000		28,474,000
Trading Securities, Equity	[3]	620,779,000		669,655,000
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[4]	$ 282,000		$ 250,000
Investment Interest Rate				8.00%
Convertible Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		5.00%
Convertible Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		12.00%
Corporate Bonds
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds	[5]	$ 0		$ 4,214,000
Trading Securities, Debt	[6]	$ 2,655,000		$ 2,477,000
Securities sold, not yet purchased, interest rate		5.55%
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		0.00%		6.25%
Securities sold, not yet purchased, interest rate				5.50%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		10.75%		13.00%
Securities sold, not yet purchased, interest rate				6.25%
Loans
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		$ 0		$ 2,209,000
Warrants and Rights
Investment Holdings [Line Items]
Marketable Securities, Consolidated Funds		3,378,000		3,000
Trading Securities, Equity		9,328,000	[3]	8,335,000
Trade Claims
Investment Holdings [Line Items]
Trading Securities, Debt		5,946,000		562,000
Mutual Fund
Investment Holdings [Line Items]
Trading Securities, Equity	[3]	$ 0		$ 6,000
Consolidated Funds | US Government Securities
Investment Holdings [Line Items]
Investment Interest Rate		0.00%		0.00%
Consolidated Funds | Corporate Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate				0.00%
Securities sold, not yet purchased, interest rate				4.38%
Consolidated Funds | Corporate Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate				14.37%
Securities sold, not yet purchased, interest rate				9.25%
Enterprise Master
Investment Holdings [Line Items]
Securities owned, at fair value		$ 1,212,000		$ 2,416,000
Enterprise Master | Preferred Stock
Investment Holdings [Line Items]
Securities owned, at fair value		487,000		1,581,000
Enterprise Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		725,000		835,000
Merger Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		801,200,000		835,700,000
Caerus Master [Member] | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value				3,200,000
Warrants and Rights | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[3]	1,900,000
Mutual Fund | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[3]			100,000
Common Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[3]	5,300,000		5,200,000
Preferred Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[3]	$ 5,500,000		$ 7,000,000
Linkem [Member] | 517210 Wireless Telecommunications Carriers (except Satellite) [Member] | ITALY | Equity Securities [Member]
Investment Holdings [Line Items]
Concentration Risk, Percentage		6.52%
Fair Value, Concentration of Risk, Investments		$ 55,030,000

[1] of September 30, 2017, the maturity was November 2017 with an interest rate of 0%. As of December 31, 2016, the maturity was March 2017 with an interest rate of 0%.

[2] As of September 30, 2017, maturities ranged from December 2017 to June 2018 with an interest rate of 0%. As of December 31, 2016, maturities ranged from February 2017 to December 2017 with an interest rate of 0%.

[3] The Company has elected the fair value option for investments in securities of preferred and common stocks with a fair value of $5.5 million and $5.3 million, respectively, at September 30, 2017 and $7.0 million and $5.2 million, respectively, at December 31, 2016. At September 30, 2017, the Company elected the fair value option for investments in warrants and rights with a fair value of $1.9 million. At December 31, 2016 the Company elected the fair value option for investments in mutual funds with a fair value of $0.1 million

[4] As of September 30, 2017, maturities ranged from February 2018 to March 2018 and interest rates ranged from 5% to 12%. As of December 31, 2016, the maturity was March 2018 with an interest rate of 8%.

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

[6] As of September 30, 2017, maturities ranged from April 2019 to October 2042 and interest rates ranged from 0.00% to 10.75%. As of December 31, 2016, maturities ranged from January 2017 to January 2036 and interest rates ranged from 6.25% to 13.00%.