Note 4 - Fair Value Measurements - Fair Value of Assets and Liabilities on a Recurring Basis (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Mar. 19, 2019	[1]	Dec. 31, 2018
Marketable securities owned		$ 73,101			$ 18,874
Other investments		14,206	$ 74,989		9,913
Fair Value, Recurring [Member]
Marketable securities owned		73,101			18,874
Equity investments		3,956
Other investments					490
Total assets:		77,057			19,364
Marketable securities sold, but not yet purchased		3,855			4,626
Total liabilities:		3,855			4,626
Fair Value, Recurring [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments					490
Fair Value, Recurring [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
Other investments	[2]
Fair Value, Recurring [Member] | Limited Partner Investment in Private Equity Fund and Real Estate Funds [Member]
Other investments	[2]
Fair Value, Recurring [Member] | Total Return Swap [Member]
Other investments		3,956
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Marketable securities owned		15,245			18,874
Equity investments
Other investments
Total assets:		15,245			18,874
Marketable securities sold, but not yet purchased		3,855			4,626
Total liabilities:		3,855			4,626
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
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	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Total Return Swap [Member]
Other investments
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Marketable securities owned
Equity investments
Other investments					490
Total assets:					490
Marketable securities sold, but not yet purchased
Total liabilities:
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments					490
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Limited Partner Investment in Private Equity Fund and Real Estate Funds [Member]
Other investments	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Total Return Swap [Member]
Other investments
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Marketable securities owned		57,856
Equity investments		3,956
Other investments
Total assets:		61,812
Marketable securities sold, but not yet purchased
Total liabilities:
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
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	[2]
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Total Return Swap [Member]
Other investments		3,956
Reported Value Measurement [Member]
Total assets:		141,846			1,352,666
Reported Value Measurement [Member] | Fair Value, Recurring [Member]
Marketable securities owned		73,101			18,874
Equity investments		3,956
Other investments					9,913
Total assets:		87,307			28,787
Marketable securities sold, but not yet purchased		3,855			4,626
Total liabilities:		3,855			4,626
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Investments In Hedge Funds Managed By HCS [Member]
Other investments					490
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Investments in Private Equity Funds Managed by HCS & JMPAM [Member]
Other investments	[2]	5,188			5,503
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Limited Partner Investment in Private Equity Fund and Real Estate Funds [Member]
Other investments	[2]	5,062			$ 3,920
Reported Value Measurement [Member] | Fair Value, Recurring [Member] | Total Return Swap [Member]
Other investments		$ 14,206

[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.