Acquisitions (Schedule of Statements of Operations (unaudited)) (Details) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue
Commissions and fees	$ 4,887,000		$ 2,591,000		$ 10,470,000	$ 4,859,000
Margin interest, marketing and distribution fees	2,125,000		3,693,000		5,419,000	7,249,000
Principal transactions	2,581,000		1,921,000		5,784,000	3,811,000
Interest income	909,000		1,186,000		2,240,000	2,359,000
Market making	615,000		410,000		1,085,000	973,000
Stock borrow / stock loan	771,000		423,000		1,215,000	1,004,000
Advisory fees	243,000		193,000		505,000	361,000
Other income	488,000		264,000		702,000	343,000
Total Revenue	12,619,000		10,681,000		27,420,000	20,959,000
Expenses
Employee compensation and benefits	6,614,000		4,476,000		13,905,000	9,004,000
Clearing fees, including execution costs	1,339,000		726,000		2,637,000	1,528,000
Technology and communications	953,000		400,000		1,934,000	822,000
Other general and administrative	401,000		1,185,000		1,255,000	1,918,000
Data processing	754,000		418,000		1,603,000	961,000
Rent and occupancy	698,000		593,000		1,425,000	1,124,000
Professional fees	744,000		902,000		1,399,000	1,785,000
Depreciation and amortization	377,000		251,000		825,000	445,000
Interest expense	88,000		31,000		164,000	52,000
Total Expenses	12,130,000		8,982,000		25,420,000	17,639,000
Income before provision (benefit) for (from) income taxes	489,000		1,699,000		2,000,000	3,320,000
Provision (benefit) for (from) income taxes	(10,000)		620,000		525,000	1,017,000
Net income / (loss)	$ 499,000	$ 976,000	$ 1,079,000	$ 1,224,000	$ 1,475,000	$ 2,303,000
Net income per share of common stock
Basic and diluted	$ 0.02		$ 0.04		$ 0.05	$ 0.08
Weighted average shares outstanding
Basic and diluted	30,587,794		30,455,962		30,521,878	30,455,962
Siebert [Member]
Revenue
Commissions and fees			$ 2,241,000			$ 4,105,000
Margin interest, marketing and distribution fees			2,783,000			5,555,000
Principal transactions			1,828,000			3,438,000
Interest income			16,000			31,000
Market making
Stock borrow / stock loan
Advisory fees			193,000			361,000
Other income
Total Revenue			7,061,000			13,490,000
Expenses
Employee compensation and benefits			2,890,000			5,725,000
Clearing fees, including execution costs			578,000			1,232,000
Technology and communications			262,000			509,000
Other general and administrative			887,000			1,272,000
Data processing
Rent and occupancy			320,000			615,000
Professional fees			447,000			949,000
Depreciation and amortization			251,000			426,000
Interest expense
Total Expenses			5,635,000			10,728,000
Earnings of equity method investment in related party			15,000			54,000
Income before provision (benefit) for (from) income taxes			1,441,000			2,816,000
Provision (benefit) for (from) income taxes			449,000			818,000
Net income / (loss)			$ 992,000			$ 1,998,000
Net income per share of common stock
Basic and diluted			$ 0.04			$ 0.07
Weighted average shares outstanding
Basic and diluted			27,157,188			27,157,188
StockCross [Member]
Revenue
Commissions and fees			$ 350,000			$ 754,000
Margin interest, marketing and distribution fees			910,000			1,694,000
Principal transactions			93,000			373,000
Interest income			1,170,000			2,328,000
Market making			410,000			973,000
Stock borrow / stock loan			423,000			1,004,000
Advisory fees
Other income			327,000			465,000
Total Revenue			3,683,000			7,591,000
Expenses
Employee compensation and benefits			1,586,000			3,279,000
Clearing fees, including execution costs			211,000			418,000
Technology and communications			138,000			313,000
Other general and administrative			298,000			646,000
Data processing			418,000			961,000
Rent and occupancy			273,000			509,000
Professional fees			455,000			836,000
Depreciation and amortization						19,000
Interest expense			31,000			52,000
Total Expenses			3,410,000			7,033,000
Earnings of equity method investment in related party
Income before provision (benefit) for (from) income taxes			273,000			558,000
Provision (benefit) for (from) income taxes			175,000			214,000
Net income / (loss)			$ 98,000			$ 344,000
Net income per share of common stock
Basic and diluted			$ 0.02			$ 0.06
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			(63,000)			(122,000)
Total Revenue			(63,000)			(122,000)
Expenses
Employee compensation and benefits
Clearing fees, including execution costs			(63,000)			(122,000)
Technology and communications
Other general and administrative
Data processing
Rent and occupancy
Professional fees
Depreciation and amortization
Interest expense
Total Expenses			(63,000)			(122,000)
Earnings of equity method investment in related party			(15,000)			(54,000)
Income before provision (benefit) for (from) income taxes			(15,000)			(54,000)
Provision (benefit) for (from) income taxes			(4,000)			(15,000)
Net income / (loss)			(11,000)			(39,000)
Total Combined Siebert (unaudited) [Member]
Revenue
Commissions and fees			2,591,000			4,859,000
Margin interest, marketing and distribution fees			3,693,000			7,249,000
Principal transactions			1,921,000			3,811,000
Interest income			1,186,000			2,359,000
Market making			410,000			973,000
Stock borrow / stock loan			423,000			1,004,000
Advisory fees			193,000			361,000
Other income			264,000			343,000
Total Revenue			10,681,000			20,959,000
Expenses
Employee compensation and benefits			4,476,000			9,004,000
Clearing fees, including execution costs			726,000			1,528,000
Technology and communications			400,000			822,000
Other general and administrative			1,185,000			1,918,000
Data processing			418,000			961,000
Rent and occupancy			593,000			1,124,000
Professional fees			902,000			1,785,000
Depreciation and amortization			251,000			445,000
Interest expense			31,000			52,000
Total Expenses			8,982,000			17,639,000
Earnings of equity method investment in related party
Income before provision (benefit) for (from) income taxes			1,699,000			3,320,000
Provision (benefit) for (from) income taxes			620,000			1,017,000
Net income / (loss)			$ 1,079,000			$ 2,303,000
Net income per share of common stock
Basic and diluted			$ 0.04			$ 0.08
Weighted average shares outstanding
Pro forma shares used to compute net income per share			30,455,962			30,455,962