Investments of Operating Entities and Consolidated Funds - Securities Owned at Fair Value (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Securities owned, at fair value		$ 543,740	$ 673,221
Securities owned, at fair value		107,482	165,916
Common Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	500,157	640,065
Securities owned, at fair value		81,509	77,702
Preferred Stock
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	12,397	9,604
Securities owned, at fair value		11,893	50,445
Warrants and Rights
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	11,781	9,604
Securities owned, at fair value		5,254	3,568
US Government Securities
Investment Holdings [Line Items]
Trading Securities, Debt	[2]	$ 1,791	$ 2,807
Investment Interest Rate		0.00%	0.00%
Securities owned, at fair value	[3]	$ 8,826	$ 34,201
Corporate Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[4]	$ 11,659	$ 4,909
Corporate Bonds | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		2.625%	0.00%
Corporate Bonds | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		12.50%	10.75%
Convertible Bonds
Investment Holdings [Line Items]
Trading Securities, Debt	[5]	$ 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		5,955	$ 5,950
Warrants and Rights | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	1,300	2,400
Common Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	5,500	5,200
Preferred Stock | Estimate of Fair Value Measurement [Member]
Investment Holdings [Line Items]
Trading Securities, Equity	[1]	$ 7,300	$ 6,000
Consolidated Funds | US Government Securities | Minimum
Investment Holdings [Line Items]
Investment Interest Rate		0.00%	0.00%
Consolidated Funds | US Government Securities | Maximum
Investment Holdings [Line Items]
Investment Interest Rate		6.125%	5.75%
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
Enterprise Master | Preferred Stock
Investment Holdings [Line Items]
Securities owned, at fair value		$ 0	$ 0

[1] The Company has elected the fair value option for investments in securities of preferred and common stocks with a fair value of $7.3 million and $5.5 million, respectively, at March 31, 2018 and $6.0 million and $5.2 million, respectively, at December 31, 2017. At March 31, 2018 and December 31, 2017, the Company elected the fair value option for investments in warrants and rights with a fair value of $1.3 million and $2.4 million.

[2] As of March 31, 2018, maturities ranged from June 2018 to August 2018 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 March 31, 2018, maturities ranged from August 2018 to April 2023 and interest rates ranged from 0% to 6.13%. 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 March 31, 2018, maturities ranged from October 2018 to February 2048 and interest rates ranged from 2.63% to 12.50%. 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 December 31, 2017, maturities ranged from February 2018 to March 2018 and interest rates ranged from 5% to 12%.