Acquisitions (Schedule of Statements of Operations (unaudited)) (Details) - USD ($)	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue
Commissions and fees	$ 4,679,000			$ 2,273,000			$ 15,149,000	$ 7,132,000
Margin interest, marketing and distribution fees	2,311,000			3,912,000			7,730,000	11,161,000
Principal transactions	2,342,000			2,327,000			8,126,000	6,138,000
Interest income	915,000			1,061,000			3,155,000	3,417,000
Market making	423,000			330,000			1,508,000	1,303,000
Stock borrow / stock loan	1,267,000			349,000			2,482,000	1,353,000
Advisory fees	305,000			211,000			810,000	572,000
Other income	333,000			290,000			1,035,000	633,000
Total Revenue	12,575,000			10,753,000			39,995,000	31,709,000
Expenses
Employee compensation and benefits	6,584,000			4,809,000			20,489,000	13,812,000
Clearing fees, including execution costs	1,270,000			798,000			3,907,000	2,325,000
Technology and communications	1,322,000			441,000			3,256,000	1,264,000
Other general and administrative	455,000			868,000			1,710,000	2,787,000
Data processing	784,000			527,000			2,387,000	1,487,000
Rent and occupancy	694,000			630,000			2,119,000	1,754,000
Professional fees	760,000			783,000			2,159,000	2,568,000
Depreciation and amortization	368,000			244,000			1,193,000	689,000
Interest expense	89,000			31,000			253,000	84,000
Total Expenses	12,480,000			9,131,000			37,900,000	26,770,000
Income before provision (benefit) for (from) income taxes	95,000			1,622,000			2,095,000	4,939,000
Provision (benefit) for (from) income taxes	(486,000)			349,000			39,000	1,365,000
Net income / (loss)	$ 581,000	$ 499,000	$ 976,000	$ 1,273,000	$ 1,079,000	$ 1,224,000	$ 2,056,000	$ 3,574,000
Net income per share of common stock
Basic and diluted	$ 0.02			$ 0.04			$ 0.07	$ 0.12
Weighted average shares outstanding
Basic and diluted	30,653,710			30,455,962			30,565,822	30,455,962
Siebert [Member]
Revenue
Commissions and fees				$ 1,925,000				$ 6,030,000
Margin interest, marketing and distribution fees				2,944,000				8,499,000
Principal transactions				2,041,000				5,479,000
Interest income				23,000				54,000
Market making
Stock borrow / stock loan
Advisory fees				211,000				572,000
Other income
Total Revenue				7,144,000				20,634,000
Expenses
Employee compensation and benefits				3,157,000				8,882,000
Clearing fees, including execution costs				617,000				1,849,000
Technology and communications				291,000				800,000
Other general and administrative				589,000				1,861,000
Data processing
Rent and occupancy				380,000				995,000
Professional fees				439,000				1,388,000
Depreciation and amortization				244,000				670,000
Interest expense
Total Expenses				5,717,000				16,445,000
Earnings of equity method investment in related party				30,000				84,000
Income before provision (benefit) for (from) income taxes				1,457,000				4,273,000
Provision (benefit) for (from) income taxes				353,000				1,171,000
Net income / (loss)				$ 1,104,000				$ 3,102,000
Net income per share of common stock
Basic and diluted				$ 0.04				$ 0.11
Weighted average shares outstanding
Basic and diluted				27,157,188				27,157,188
StockCross [Member]
Revenue
Commissions and fees				$ 348,000				$ 1,102,000
Margin interest, marketing and distribution fees				968,000				2,662,000
Principal transactions				286,000				659,000
Interest income				1,038,000				3,363,000
Market making				330,000				1,303,000
Stock borrow / stock loan				349,000				1,353,000
Advisory fees
Other income				351,000				816,000
Total Revenue				3,670,000				11,258,000
Expenses
Employee compensation and benefits				1,652,000				4,930,000
Clearing fees, including execution costs				242,000				659,000
Technology and communications				150,000				464,000
Other general and administrative				279,000				926,000
Data processing				527,000				1,487,000
Rent and occupancy				250,000				759,000
Professional fees				344,000				1,180,000
Depreciation and amortization								19,000
Interest expense				31,000				84,000
Total Expenses				3,475,000				10,508,000
Earnings of equity method investment in related party
Income before provision (benefit) for (from) income taxes				195,000				750,000
Provision (benefit) for (from) income taxes				4,000				218,000
Net income / (loss)				$ 191,000				$ 532,000
Net income per share of common stock
Basic and diluted				$ 0.03				$ 0.09
Weighted average shares outstanding
Basic and diluted				6,152,500				6,152,500
Pro Forma Adjustments (unaudited) [Member]
Revenue
Commissions and fees
Margin interest, marketing and distribution fees
Principal transactions
Interest income
Market making
Stock borrow / stock loan
Advisory fees
Other income				(61,000)				(183,000)
Total Revenue				(61,000)				(183,000)
Expenses
Employee compensation and benefits
Clearing fees, including execution costs				(61,000)				(183,000)
Technology and communications
Other general and administrative
Data processing
Rent and occupancy
Professional fees
Depreciation and amortization
Interest expense
Total Expenses				(61,000)				(183,000)
Earnings of equity method investment in related party				(30,000)				(84,000)
Income before provision (benefit) for (from) income taxes				(30,000)				(84,000)
Provision (benefit) for (from) income taxes				(8,000)				(24,000)
Net income / (loss)				(22,000)				(60,000)
Total Combined Siebert (unaudited) [Member]
Revenue
Commissions and fees				2,273,000				7,132,000
Margin interest, marketing and distribution fees				3,912,000				11,161,000
Principal transactions				2,327,000				6,138,000
Interest income				1,061,000				3,417,000
Market making				330,000				1,303,000
Stock borrow / stock loan				349,000				1,353,000
Advisory fees				211,000				572,000
Other income				290,000				633,000
Total Revenue				10,753,000				31,709,000
Expenses
Employee compensation and benefits				4,809,000				13,812,000
Clearing fees, including execution costs				798,000				2,325,000
Technology and communications				441,000				1,264,000
Other general and administrative				868,000				2,787,000
Data processing				527,000				1,487,000
Rent and occupancy				630,000				1,754,000
Professional fees				783,000				2,568,000
Depreciation and amortization				244,000				689,000
Interest expense				31,000				84,000
Total Expenses				9,131,000				26,770,000
Earnings of equity method investment in related party
Income before provision (benefit) for (from) income taxes				1,622,000				4,939,000
Provision (benefit) for (from) income taxes				349,000				1,365,000
Net income / (loss)				$ 1,273,000				$ 3,574,000
Net income per share of common stock
Basic and diluted				$ 0.04				$ 0.12
Weighted average shares outstanding
Pro forma shares used to compute net income per share				30,455,962				30,455,962