Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Investment Holdings [Line Items]
Securities owned, at fair value		$ 673,221		$ 700,876
Securities owned, at fair value		165,916		79,237
Common Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	640,065		669,655
Securities owned, at fair value		77,702		28,474
Preferred Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	9,604		15,811
Securities owned, at fair value		50,445		37,343
Warrants and Rights
Investment Holdings [Line Items]
Trading Securities, Equity		9,604	[1]	8,335
Securities owned, at fair value		3,568		3
US Government Securities
Investment Holdings [Line Items]
Trading Securities, Debt	[2]	$ 2,807		$ 3,780
Investment Interest Rate		0.00%		0.00%
Securities owned, at fair value	[3]	$ 34,201		$ 6,994
Corporate Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[4]	4,909		2,477
Securities owned, at fair value	[5]	$ 0		$ 4,214
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		0.00%		6.25%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		10.75%		13.00%
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[6]	$ 282		$ 250
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%
Trade Claims
Investment Holdings [Line Items]
Trading Securities, Debt		$ 5,950		$ 562
Mutual Fund
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	0		6
Loans
Investment Holdings [Line Items]
Securities owned, at fair value		0		2,209
Warrants and Rights | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	2,400
Mutual Fund | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]			100
Common Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	5,200		5,200
Preferred Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	$ 6,000		$ 7,000
Consolidated Funds | US Government Securities
Investment Holdings [Line Items]
Investment Interest Rate				0.00%
Consolidated Funds | US Government Securities | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		0.00%
Consolidated Funds | US Government Securities | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		5.75%
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		$ 608		$ 2,416
Enterprise Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		608		835
Enterprise Master | Preferred Stock
Investment Holdings [Line Items]
Securities owned, at fair value		$ 0		$ 1,581

[1] The Company has elected the fair value option for investments in securities of preferred and common stocks with a fair value of $6.0 million and $5.2 million, respectively, at December 31, 2017 and $7.0 million and $5.2 million, respectively, at December 31, 2016. At December 31, 2017, the Company elected the fair value option for investments in warrants and rights with a fair value of $2.4 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

[2] As of December 31, 2017, maturities ranged from February 2018 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] of December 31, 2017, maturities ranged from January 2018 2017 to April 2022 and interest rates ranged from 0% to 5.75%. As of December 31, 2016, the maturity was March 2017 with an interest rate of 0%.

[4] As of December 31, 2017, maturities ranged from October 2018 to May 2040 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%.

[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 December 31, 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%.