Note 4 - Fair Value Measurements - Fair Value of Assets and Liabilities on a Recurring Basis (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Mar. 19, 2019	[1]	Dec. 31, 2018
Marketable securities owned		$ 92,190			$ 18,874
Other investments:
Other investments		13,530	$ 74,989		9,913
Marketable securities owned		92,190			18,874
Fair Value, Recurring [Member]
Marketable securities owned		78,557			18,874
Other investments:
Equity investments		3,568
Other investments					490
Total assets:		82,324			19,364
Marketable securities sold, but not yet purchased		2,696			4,626
Total liabilities:		2,696			4,626
Marketable securities owned		78,557			18,874
Fair Value, Recurring [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments:
Other investments		199			490
Fair Value, Recurring [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Investments In Funds Of Funds Managed By HCS [Member]
Other investments:
Other investments	[2]	199			490
Fair Value, Recurring [Member] | Limited Partner Investment in Private Equity Fund and Real Estate Funds [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Total Return Swap [Member]
Other investments:
Other investments		3,767
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Marketable securities owned		15,717			18,874
Other investments:
Equity investments
Other investments
Total assets:		15,717			18,874
Marketable securities sold, but not yet purchased		2,696			4,626
Total liabilities:		2,696			4,626
Marketable securities owned		15,717			18,874
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments:
Other investments
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Investments In Funds Of Funds Managed By HCS [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Limited Partner Investment in Private Equity Fund and Real Estate Funds [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Total Return Swap [Member]
Other investments:
Other investments
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Marketable securities owned
Other investments:
Equity investments
Other investments					490
Total assets:		199			490
Marketable securities sold, but not yet purchased
Total liabilities:
Marketable securities owned
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments:
Other investments		199			490
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Investments In Funds Of Funds Managed By HCS [Member]
Other investments:
Other investments	[2]	199			490
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Limited Partner Investment in Private Equity Fund and Real Estate Funds [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Total Return Swap [Member]
Other investments:
Other investments		199
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Marketable securities owned		62,840
Other investments:
Equity investments		3,568
Other investments
Total assets:		66,408
Marketable securities sold, but not yet purchased
Total liabilities:
Marketable securities owned		62,840
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments:
Other investments
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Investments In Funds Of Funds Managed By HCS [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Limited Partner Investment in Private Equity Fund and Real Estate Funds [Member]
Other investments:
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Total Return Swap [Member]
Other investments:
Other investments		3,568
Reported Value Measurement [Member]
Other investments:
Total assets:		158,394			1,352,666
Total liabilities:		87,125			1,223,794
Reported Value Measurement [Member] | Fair Value, Recurring [Member]
Marketable securities owned		78,557			18,874
Other investments:
Equity investments		3,568
Other investments					9,913
Total assets:		92,087			28,787
Marketable securities sold, but not yet purchased		2,696			4,626
Total liabilities:		2,696			4,626
Marketable securities owned		78,557			18,874
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments:
Other investments		199			490
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
Other investments:
Other investments	[2]	5,045			5,503
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Investments In Funds Of Funds Managed By HCS [Member]
Other investments:
Other investments	[2]	5,244			5,993
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Limited Partner Investment in Private Equity Fund and Real Estate Funds [Member]
Other investments:
Other investments	[2]	4,718			$ 3,920
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Total Return Swap [Member]
Other investments:
Other investments		$ 13,530

[1] The fair value of the Company's retained interest in CLO III, CLO IV, CLO V, CLO VI, and JMPCA as of the deconsolidation date was $13.3 million, $27.8 million, $26.5 million, $3.8 million, and $3.6 million, respectively

[2] In accordance with ASC 820-10, certain investments that are measured at fair value using the net asset value per share (or its equivalent) have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the statement of financial position. The carrying values of these lines reconciles to the parenthetical disclosure of other investments on the Consolidated Statements of Financial Condition.