Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) - USD ($) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2018		Dec. 31, 2017
Investment Holdings [Line Items]
Securities owned, at fair value		$ 644,169		$ 673,221
Securities owned, at fair value		108,901		165,916
Common Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	576,720		640,065
Securities owned, at fair value		55,535		77,702
Preferred Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	6,466		9,604
Securities owned, at fair value		24,314		50,445
Warrants and Rights
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	21,403		9,604
Securities owned, at fair value		4,848		3,568
US Government Securities
Investment Holdings [Line Items]
Trading Securities, Debt	[2]	$ 2,842		$ 2,807
Investment Interest Rate		0.00%		0.00%
Securities owned, at fair value	[3]	$ 20,446		$ 34,201
Corporate Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[4]	30,992		4,909
Securities owned, at fair value		$ 3,758	[5]	$ 0
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		0.50%		0.00%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		14.00%		10.75%
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[6]	$ 0		$ 282
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		5,746		$ 5,950
Warrants and Rights | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	1,100		2,400
Common Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	6,200		5,200
Preferred Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	$ 2,900		$ 6,000
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
Investment Holdings [Line Items]
Investment Interest Rate				5.88%
Enterprise Master
Investment Holdings [Line Items]
Securities owned, at fair value		$ 469		$ 608
Enterprise Master | Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value		$ 469		$ 608

[1] The Company has elected the fair value option for investments in securities of preferred and common stock with a fair value of $2.9 million and $6.2 million, respectively, at September 30, 2018 and $6.0 million and $5.2 million, respectively, at December 31, 2017. At September 30, 2018 and December 31, 2017, the Company elected the fair value option for investments in warrants and rights with a fair value of $1.1 million and $2.4 million.

[2] As of September 30, 2018, maturities ranged from October 2018 to August 2019 with an interest rate of 0%. As of December 31, 2017, maturities ranged from February 2018 to June 2018 with an interest rate of 0%.

[3] As of September 30, 2018, maturities ranged from November 2018 to December 2018 and interest rates were 0%. As of December 31, 2017, maturities ranged from January 2018 to April 2022 and interest rates ranged from 0% to 5.75%.

[4] As of September 30, 2018, maturities ranged from October 2018 to April 2049 and interest rates ranged from 0.50% to 14.00%. As of December 31, 2017, maturities ranged from October 2018 to May 2040 and interest rates ranged from 0.00% to 10.75%.

[5] As of September 30, 2018, the maturity was December 2023 with an interest rate of 5.88%.

[6] As of December 31, 2017, maturities ranged from February 2018 to March 2018 and interest rates ranged from 5% to 12%.